Although I've been plunking around with claw-hammer 
banjo playing for several decades, I feel I never got 
very far largely because that nearly every time 
I'd drop my thumb to play a note on any of the first 
four strings (called "double-thumbing"), my darn thumb-nail, 
even if cut short, would obnoxiously catch on a metal 
string & goof up the tune. Nothing I ever tried eliminated 
this problem, which caused me to pretty much give up 
banjo playing.

I've FINALLY found the answer --- NYLON banjo strings! 
For some unknown reason, my thumb-nail does not even start 
to catch when using nylon strings & I now am having a blast 
working out a lot of the old-time tunes (that I've played 
forever on the fiddle) on the banjo. 

In short - putting nylon strings on my 5-string banjo made 
ALL the difference in the world as far as double-thumbing & 
banjo playing in general. An added benefit is that (for me 
at least) the instrument also sounds better, plunkier & 
somehow more down to earth, using nylon.

Give nylon strings a try if for any reason you have any 
trouble double-thumbing, It made all the difference in 
the world in my case.