Hello.
I have had a few sticky fire buttons in the past myself, and each time I have found electrical contact cleaner to be a godsend. You can get it in aerosol spray bottles- spray this over, and into the fire button, and allow it to dry out.
Obviously remove your batteries when you do this though, and allow it to dry out thoroughly before inserting them in again.
If this doesn't work, and it is physically catching on something, you may have to open the mod if you feel comfortable. I had to do this with a IPV D2. I repositioned the button, and while I was in there gave it a more thorough clean. This did work.
I hope you can get the issue sorted soon.
