I have read several posts here where people mention that their roller ball tip is not working right.
I have came across this problem too while using some roller ball tips.

I have a solution to this I thought I would share.

It seems to me that that there is a tiny air bubble that gets lodged up near the roller ball tip and that's why sometimes it doesn't work.
The quick fix to this, I have found, is to turn the bottle upside down (with the cap on firmly) and give it a quick and firm shake. No need to keep shaking it up and down, up and down. One good flick is all it takes.
And Presto...Your roller ball tip works!

I hope this helps!