What do you mean wont go into gear....does it make a graunching sound?
Idle must be a low as possible. Also try pushing the bike when engaging drive to help the dogs engage without graunching the dogs. Cable adjustment is important there should be about 1mm free play in the cable when in the engaged position. The engagement is achieved by a spring behind the sliding gear with dogs. If the spring is sacked out it may not be pushing hard enough or if the dogs are rounded off it may not stay engaged.
Let us know more of the symptoms.