Hello,
Please take a look at your club facilities information (Club Screen > Information > Facilities), there is an information about the level of your youth facility. If you find it at low level, you can ask your boards to upgrade your youth facility.
Also, you have to sign some coaches who best at youth training, they're the key part of youth development.
Check your youth training schedule, make sure it's not too high and low, you also can see at your youth players satisfaction about training schedule, ensure that they're happy or satisfied with your training schedule.
You can also warn your youth players who don't develop their playing skill (be careful of using this, your player might accept your critic or even refuse it and go angry, always save your game before doing this so you can load your game back when they go unhappy). (Player screen > Development > Training)
If you currently have top youth training facility, good youth players would likely to come to your youth squad when your main team won several tournaments, they will come in certain year.
Go back to your youth team training schedule, you can switch the tab to "Coaches" and look at the coaching intensity, you can manually arrange your training coaching into the best quality until all coaches' workload have highest star (5 stars if possible).
Also, always use squad rotation in your youth squad to prevent tiredness and injuries, those both can decrease your player developments.
Hope it helps, CMIIW,
thanks.