As an American living in the UK (4 years now), my opinion is that the two major reasons for failure are your courts and your coaches.
Most clubs have all-weather courts (carpet with sand on them), which play nothing like any courts used in professional tournaments. At best, they can almost simulate grass, which is by far the least used surface for ATP tournaments. They also tend to throw up a lot of unpredictable bounces, which prevent kids from learning to successfully hit on the rise or return serve aggressively.
Those clubs that have hard courts can't play too often because of the constant rain. So, the only people who get constant training on good courts are those whose clubs have indoor facilities, which limits the talent pool quite a lot.
On top of that, the coaching I've seen around here sucks. I got better coaching when I was a teenager from cheap group lessons than I see the kids around here getting from private lessons. The number of juniors at my club with major flaws in their serves despite all the coaching I've seen them get these 4 years is frankly appalling. They have major hitches and other problems that the coaches allow to continue year after year.
Having seen some of the crap players from my club who have been able to get their LTA coaching certificates, I'm not surprized. We have one guy who is certified despite not knowing how to serve properly (he uses a forehand grip on a push serve like a begginer), amongst other things; of course he won't be able to teach a kid how to play well. We also have a lady who can't volley to save her life and struggles to sustain a decent rally. How good of a coach is that?
So, in my opinion, you've got two options.
1. Send all your kids to Spain like Dandy Andy
2. Fire all your coaches and replace all your all-weather courts with hard or clay courts and put up roofs, tents, or some sort of cover over them to block the rain. They don't need to be fully enclosed indoor courts; they just need to be dry.