It is very possibly you BUT....... sometimes client's come in and say "it just fell off"...
and the truth is that they helped it fall off.
To be sure it wasn't you, put the hand under the UV light.
If the nail 'fell' off, you won't see tell-tale damage.
IF IT WAS HELPED OFF.. you'll see tell-tale damage of the nailplate, it'll look 'white'. Nails that are pried/bitten/forced off will rip the surface of the nail plate since they were well adhered, and when forcing them off, it will tear the nail plate if they are well stuck.
IF they are not well adhered due to prep or whatever... then there will be no damage to find.
ALSO even if a client did not help it off forcibly, liberal use of babyoil/mineral oil, petroleum jelly/petrolatum can greatly cause lift.
So, certainly rule out any errors in your prep & application. Double, triple and quadruple check your methods.
And yes, as a newbie, there will be hiccups in the road and a 'learning curve'. But it's also important to know that there are other things that will 'help' nails 'fall off', other than you.
Do a search on here regarding lift, you'll find a multitude of threads on it. Knowledge will arm you appropriately in this battle against lift and other such problems.
hth's