This thread could go on with all the guesses....too wet mix ratio......humidity....cold hands.
When basically the answer is you have no training in how to use your products and you need to get some pronto!!
It really is as simple as that.
Training 15 years ago.....for the Nail Industry that was like the Dark Ages!!!
Even just 10 years ago I was taught to etch the nail with a rough abrasive and mix ratio was never mentioned, just pick up a bead and then pat and wipe until you had covered the nail. No structure, mentioned. I was also not taught any filing techniques, and would just file until I got a smooth surface, again no finished structure mentioned.
Possibly your college training has given you some knowledge of nail anatomy and Health & Safety issues. So you maybe don't need to do a beginners course maybe a 1 or 2 day refresher, but you do need to do something, and not just 'wing it' on the basis you did a course 15 years ago.
Products have improved so much, and the emphasis now is on the health of the natural nail (as it should be)
Education in nail enhancements has come a long way in the last 10 years, and the best nail technicians are constantly updating their skills and IMHO everyone should attend a training course at least once a year in order to keep up to date with products and techniques.
I hope this hasn't come across as harsh, I just wanted to be honest.
The practicalities are, you can ask as many questions on here as you like, but we all use different products and have different techniques in relation to those products. You need to pick a product company and get some training direct from them. Then this site will become of great use to you as you can be more specific with your questions, and likewise geeks with the answers.
Best of luck.