How are you applying the Shellac. What are your steps? Do you have any problems with other clients?
I've not heard of medications altering Shellac's longevity. (That's not saying it's impossible.) I have a client/friend who wears enhancements, and she's severely diabetic (uses an insulin pump). And numerous clients who are on medications for HBP.
I was told it wasn't completely impossible for a client's nails to be too oily to hold Shellac. But, that there haven't been any confirmed instances.