IMO you will never learn prepping on the nailtrainer, and as proper PREP is such a huge part of the product not lifting, then it is hard.
Also the producs can play a role, of course there are inferior products, but most important thing to remember is that every system has its own way of application, mix-ratio etc. which means that to get the proper result a conversion class as mentioned above is a key factor in learning nails.
Unfortunately, as far as I know the Essential nailcourse gives you zero recognition when it comes to some of the greatest companies, (it used to be that way) which means you will have to start all over. This will cost a bit more, but will be well wort it.
The nailtrainer is a great tool, when you use it to practise the perfect techniques, but unfortunately it is not able to correct you, an a lot of bad habbits can be developed. The trainer doesn't care if you overfile the nails, it never tells you if you are filing to fast and cause a burn in the nails, it doesn't use the fingers so mix-ratio is very forgiving, it's not hot in any way, so you get use to the prouct you use is on a cold surface, which will make it perform differently for you when put on a hotter surface and I could go on.
I used to buy the class, never send it in, didn't bother in the end as comming on here made me realize that CND was a perfect solution for me: I was shown all proper ways in classes, and because I had both real hands AND my trainer hand I really got a feel of how my product worked in my environment and could do the training at my own pace at home.