It could be that the bonds were not seller correctly and water or oily products have broken them down and left a trace of glue. Check the bonds for cracks or ridges from top to bottom, this shouldn't be visible if the bonds were correctly rolled and sealed hope this helps x
You need to roll the bond along the full length of your finger and thumb to ensure that it is done correctly.
If your client has naturally oily hair then nail tips aren't suitable for her. Usage of the correct aftercare is essential as it won't damage the bonds.
It is normal for 1 or 2 to fall out though.