There are a couple of reasons you might be having this problem. First, make sure your laptop does not have a "disable mouse button" command. This feature can usually be turned on and off by pressing Fn+(F1,F2,F3, etc). If you still have the user manual you can check that for the specific command. Another reason is a bit more intensive to check. The ribbon cable connecting the touchpad buttons to the motherboard might be disconnected, which would require you to remove the bottom cover of the laptop to reinsert. Removing the bottom plate is fairly easy, just follow the guide here. The cable you'd be looking for would be coming out of the back of the touchpad and leading to the motherboard. The buttons themselves could also be faulty. This could happen over time through use, or if something got spilled on them. In this case your best option would be to replace or clean them.