I had the same problem with an otherwise good laser mouse (with a second mouse from me and a third mouse from a former girlfriend also, the same model).
Sometimes I had to press hard, sometimes the button bounced.
I disassembled it countless times, glued something on it, took it away again, put a drop of ethanol into the micro switch (You can try it, but let it dry!). It worked for a few months at a time.
The only thing that really helped was a new mouse.