Apart from something causing a barrier on the eyelash I have no idea. If the client was using eye ointment or something similar then that could prevent it taking. I know some glaucoma meds are quite strong.
Maybe the solution is out of date.
I use LVL which comes in separate sachets. If you damage the package then the oxygen will get in and make the product ineffective.
I prefer the individual sachets as there is less chance of them going 'off'.
In the bottles they are constantly exposed to oxygen and so don't last as long.