When I use manual gear shift, it works for the first three gears easily. However, when I try it with the last two/three gears, it doesn't work. Is there a way to fix this?

- is the car you are driving the Formula BMW (FBM)?
- if so, then the answer is that it has a sequential gearbox without ignition cut*, and generally, you should lift throttle every time your change gears. It is not a bug or glitch, it's how the car's gearbox works.