There can be a number of different causes.
Does the lever feel like it's loose, front to back? If so, the switch may be coming apart. Fix: replace TS switch.
Does one side work correctly but the other doesn't? Easy test: turn on the hazard flashers. Check all of your bulbs. If they are all flashing on hazard but using the turn on one side still isn't right, the fix is likely a TS switch. If one (or more) is not working and it is (they are) on the same side that doesn't work correctly, replace the bulbs (or swap from one side to the other) and see if that fixes the issue. If one side flashes normally and one side flashes more rapidly, you either have a bulb not working (burned out, bad contact, wire severed, etc.) or you have too much load on that side (trailer hooked up, short to another circuit, etc.). Modern flashers are designed to flash the proper load. If you have 2 bulbs to flash on each side, the TS flasher wants to "see" 2 bulbs, or at least the load it expects. If you add or remove bulbs, or if you switch to LED bulbs, the flash rate will change until the load is restored to its designed value. For LED bulbs that's easy - add load resistors. For burned bulbs that's easy - replace them. For other issues, you'll need to do some diagnosis.
PM me if you need more input from me, or post here for more help from all of the members. They're quite a great group of folks!