how to fix bubbles in felt on flat roof?

I have a small building extension with a flat roof roofed with felt and which has two or three "bubbles", how can these bubbles be fixed?
I think I read somewhere that I could make a couple of cuts in the felt in the shape of an X and then remove excess felt when bubble flattened out.
Then seal the cuts.
Does this work?
Any help/advice appreciated - Thanks