This year’s Vuelta has been one of the poorest for Spain. They got three stage wins at least, but only two in the top 20 and none in the top ten. 1996 was a disaster too, only saved by Escartin’s 10th place on GC (and five in top 20). But 1996 was without a single Spanish stage win.
Edit: wrote two stage wins first. Forgot about Soler!
2017 I think was the nadir. It took until stage 20 to get a stage win, and they only had one man in the top 10. Both the stage winner and the top 10 were a near 35-year-old Alberto Contador who was retiring after the race. The next best performers for Spain were a 33yo Sérgio Pardilla, a career-best 15th as he topped out at a top 20 rider in GTs, a 33yo Mikel Nieve in 16th, Daniel Moreno in 18th, who turned 36 during the race, and then in 22nd - I kid you not - 32yo sprinter José Joaquín Rojas. Pello Bilbao in 23rd was the first Spaniard under 30 in the race, sandwiched between Rojas and 33yo Luís Ángel Maté. The only Spanish up-and-comer was Jaime Rosón in 26th, a performance for which he'd later be suspended under biopassport irregularities.
I think a problem for that generation is really that the bottom almost completely fell out of Spanish cycling's support infrastructure in the early 2010s following the financial crisis, and with the glut of Puerto exiles hogging the strong salaries in Portugal on the way in, that route was restricted in access too. With far fewer pro teams and opportunities, riders were staying amateur longer, or pursuing very circuitous routes through the Asia Tour, which usually required slightly older riders willing to take that risk (take Edu Prades and Jon Aberasturi, both born in the late 80s, rather than the youngsters of the time born in the early 90s). Big hopes were appended to Avenir winners in Rubén Fernández and Marc Soler, neither of whom panned out as GC riders, and then you had the likes of Jaime Rosón and Xuban Errazkin who both fell foul of anti-doping. Guys like Óscar Rodríguez and Sérgio Samitier are a decent backup squad but they aren't leaders. Mas was pulled from Contador's amateur team to Klein Constantia, a short-lived development team for Quick Step, at age 20, which probably helped his coming through as he wasn't left to stagnate in the ailing Spanish amateur system of the time for too long as we did see a fair few riders of that era turn into decent riders, but usually a few years later on down the line as late bloomers and again, more into support roles.
And I suppose the other issue that you could argue is that the fact they came off the back of an extremely strong generation, many of whom continued to secure excellent results deep into their 30s, meant that opportunities to lead were restricted for the Spanish riders coming through at that time. Obviously Alejandro Valverde is the standout here, anomalous even among this group for his longevity, but Joaquím Rodríguez, Dani Moreno, Luís León Sánchez, Samuel Sánchez and Alberto Contador dominated leadership roles among Spanish riders and teams taking on large Spanish contingents for many years, while simultaneously even large parts of the support squads and minor team leaders had great longevity, with the likes of Rubén Plaza, Mikel Nieve, Dani Navarro, Sérgio Pardilla and David de la Cruz.
Parallel to this, once Spanish cycling's infrastructure started to rebound, with the number of teams rising again and prospects coming through, that mid-90s generation was rather forgotten about as the talents coming through from a younger generation were always going to be more attractive to pro teams than potential diamonds in the rough. After all, we saw a situation like in 2011-12 when Arkaitz Durán went back into the amateur scene at around 25 and absolutely dominated only to go back to mediocrity as a pro in Portugal, because his endurance was somewhat limited and while he could be dominant in the 120-150km region, longer stages saw him become progressively less competitive - and a few late bloomers scoring big results as pros being guys like David Belda left a few teams wary of these late bloomers unless they had a backstory to explain it, like late converts to the sport or transfers from other sports. And in parallel to this, as the scene became more well-developed again, we saw an increase in riders from elsewhere going to race in Spain for their development, and it is notable that quite a few of the riders who've turned pro from the Spanish amateur scene in recent years, even with the Spanish teams, have not been Spanish - the likes of Abner González, Sérgio Chumil, Guillermo Thomas Silva coming - as many of them tend to - from Latin America (some of the top figures on the Spanish domestic scene at present are also from this wider region), but also Britons, Belgians, Caribbeans, Russians and US-Americans have been finding their way through the espoir system in Spain - leaving fewer spots for Spanish late-bloomers to pick up pro spots.
The most successful rider to come out of the Spanish amateur scene to have been born in the first half of the 90s would almost certainly be Richard Carapaz, who Movistar put through some paces at Lizarte after signing him up from Colombia (he was riding for the Colombian team Bicicletas Strongman, before anybody pulls me up on that). Obviously 99,9% of the péloton would gladly swap palmarès with a guy who's won a Giro, been on the podium of all three Grand Tours, won stages of all three Grand Tours too, and won Olympic gold - but of course while he may be at least partly a product of the Spanish cycling system, coming through Lizarte and Movistar... he isn't Spanish.