Try this:
Go to options > controls
1. Choose Shifter Layout Behaviour: Advanced
2. Then choose one of these options: range+split, zf, scania 12 speeds, volvo etc...
3. Go back to game and try to change gear.
if it doesn't work go back to step 2 and choose different options. Try 3-4 options and then if it doesn't work, go back to options and reset the clutch hotkey and bind it again.