I just love it when a client says "It just popped off"
I have been doing nails for near on 20yrs but only qualified recently and in al my time of doing nails on myself and doing them as dodgy as they come, I have never had a nail just pop off.
I have even gone out of my way to do a really bad set, using no primer, bad mix ratio and no prep work and the nails lasted with no infills for over 6 weeks and they didn't budge one bit.
So I find it amazing that client say their nail just pop off.
It normally comes down to prep work and bad sizing with tips as some of the big culprits, if this is actually happening.
Are you both using the same batch of product?
How old is the product?
Is it good quality product?
Maybe it has seen better days and needs replacing cause the primers is too old or the L&P is too old and not adhering correctly.
Are you both making the nails not too thin?
Is your mix ratio right or do you both struggle with it?
As already suggested, are you using tips, are you sure you are both fitting them correctly. Were you taught to use a smaller sized tip V's a larger sized tip if it doesn't exactly fit, if you were taught to go smaller this could be your problem.....it puts too much pressure on the enhancement, as the product sets it shrinks slightly, then add a tip that is already too small and you could imagine what could happen.
Are you making sure you are wiping all cuticle eraser off thoroughly and dehydrating the nail really well before going into your application?
You would be surprised that one small step not being done correctly, can make all the difference.
Maybe if you tell us your all your steps and what products/brand you use, we might be able to help more.