How many times do you hear the cry " i wasn't doing anything - it just pinged off!" giving me wonderful visions of people merrily watching Eastenders sat with their hands in their lap and with true Harry Potter proportions their nail just pinging across the room with absolutely no explanation other than some mystical power causing it.
Nah! More than likely if it is happening to only a few of your client base they are telling big stonking porky pies. I had the same problem as you with one client - she would come back fortnightly with half of the nails missing and wouldn't admit to anything. I would check and double check my prep work, make sure I'd not gone too close to the edges etc.and still to no avail Anyway, I eventually coaxed out of her that when she was drunk she would pick them off!
Now if I come against such a client I use the empathetic approach - during their treatment I will casually drop into conversation how when I feel stressed, or when I have a little edge, or a little lifting etc etc I just can't resist picking or biting at it! Once you've "confessed your sins" you'll be amazed how people suddenly find religion and get all confessional and join in!
Or some people don't even know they are doing it - one lady constantly had little chunks out of the corner of her nails but swore blind that she didn't do it. One day her daughter came with her and we went through the same old consultation. The next time she came she said "you were right my daughter caught me nibbling when I watch the telly and I had no idea I was doing it"
CASE CLOSED !!!!
Finally if they have picked or knocked it off it usually leaves scuff marks on the nail plate, if they really did just come off the surface is completely smooth and may mean their nail plates are naturally too oily to allow the product to adhere properly.
Sorry this is long winded but hope it helps.
Sarah