RevoShift Friction: shifting to the largest gear takes a lot of force?
Some of the revoshifts are actually friction and just don't click period, while others are pseudo-friction (they click in small increments so you can trim your shifts) - I suspect you have the latter. They are also the bargain bin parts (less than 20 dollars for a set), and thus are built with low quality and things often go wrong. Pretty much all you can do is check that the cable tension is OK, the cable isn't caught on anything and the derailleur is set up properly (If the shifter is gunked up, sometimes flushing it with WD40 helps, but thats just a temporary fix).

As suggested in this question, I recommend throwing out the RevoShift shifters if they start to cause problems and then replacing them with some other cheap shifters (maybe switch the left on a cheap thumb friction shifter, or a Shimano trigger shifter with the same number of speeds, or a SRAM Attack front shifter if you want to stick with twist).