Poor tackling & Gap-8

Wow its funny how many similar problems other coaches have that i have on this site....i recently posted some questions on here pertaing to some good tackling drills and or defenses that could help me out. I stumbled on the Gap-8 and thought it would be perfect for the team we played tonite, or any team for that matter. The scheme was flawless, we had penetration like we havent had all season...but when it came to making the tackle they went high at the shoulderpads and didnt wrap up. and that has been our achilles' heel all season, we should have shut so many teams out yet all the touchdowns they have scored have been off missed and terrible tackles IN THE BACKFIELD and they always end up a TD. and this is killing me, because we continually do tackling drills in practice, and not just the same one, different ones to simulate different aspects. Yet when we get to game time no one shows up, in practice we have headhunters left and right, but we have 4 main heavy hitters who can tackle...IN PRACTICE. when we get to the game it's like they forgot everything we have tought them and now we are 1-4 and i dont know what to do. This is my first time as an actual coach and im not taking these losses so well. Another thing is though its that when the times get tough in games these kids are giving up, and it isnt a reflection of us coaches because i try and stay positive throught the entire game. The poor tackling is killing me and i need help, can someone help me with my problem? er problems? o and btw the Gap-8 is my new favorite defense ( tonite we played the undefeated team in our division who ususally averages 5-6 TD's on teams but we stopped them at 3, a huge plus even after the fact our kids gave up and were still tackling horrible) so for Coach J, it's not the scheme buddy but i feel your pain...somebody plz reply with help a.s.a.p! thanx

- Coach J-Mac