I can understand where you are coming from Hun. I used to have loads of problems due to prep. I believed mine was spot on, but when a good friend of mine who is a tech, watched me do it, she could see minor problems with what i was doing.
Things like, not removing the dust after taking off the shine from the sidewalls. Plus, i wasn't pulling back the skin of the sidewalls, to make sure all shine was removed properly. I also found i over primed, which can also cause lift badly.
These are all minor things, but they can cause you to have terrible lift.
The other thing i would say is how many full sets or in fills have you done? I have been doing nails for over three and a half years and its only in the last year that i have truly sorted all of my lifting problems, mix ratio and application techniques. Becoming a good nail tech takes LOADS of practice, loads of experience, and lots of re-training and patience. Believe me i know. Even now i am always striving to do better and be a better tech.
3 months is no where near long enough to really know what you are doing. I hope i don't sound condescending, i really don't mean to, but you should get a techie friend to see how you do your nails and they should see any reasons for problems etc. If not you should do some trouble shooting courses with Creative or Young nails. Keep at it Hun, you will get there. Believe me there have been times a long the way where i wanted to throw in the towel, but when a client you've had problems with comes in for in fills and has no lift, there is nothing more satisfying.