Assuming the shifter is in good condition, be sure the shifter and derailleur match -- both 6 speed or 7 speed. 7 speed is most common but there are a few 6 speed RevoShifts around. If the RevoShift is 7 speed but the derailleur isn't, it won't index shift to the smallest or largest cog. (But many rear derailleurs can squeeze out an extra "gear" in friction mode, so I can cover my 8-speed cassette with a 7-speed Exage or Deore long cage RD in friction mode with bar end or thumb shifters.)

A typical matchup on many entry level hybrids would be RevoShift grip shifters and Shimano Tourney or Altus RD. Those should work fine together if everything is in functional condition.

Otherwise, the RevoShift grip shifter isn't serviceable. It's 99% plastic with a couple of bits of sheet metal and springs. If the derailleurs, cables, etc., all check out, the shifter is probably broken and needs to be replaced. Been there, done that.

My Globe Carmel came with RevoShift. Works fine for as long as it lasts, but can't really be serviced. It started with difficulty shifting to one extreme, then became harder overall to shift. I dismantled it and realized (1) it's very difficult to disassemble without marring the soft plastic latches that hold it together, and (2) it can't be repaired anyway.

I bought some spare RevoShift kits for about $10 or less as spares. The kits include everything -- grip shifters, cables and housings -- but the cables and housings aren't quite long enough for some long wheelbase comfort hybrids like my Globe Carmel, so I used some spare cables/housings from other projects.

If you get tired of trying to make it work, try SunRace SLM10 thumbies. They're cheap and good, an uncommon value. Just friction shifting but sometimes a better solution than trying to fix a balky RevoShift or comparable low end grip shifter.