Most likely the starter solenoid is not "kicking out" to engage the flywheel. When it does not start does the starter spin? if so good time to invest in a new starter or, if you can pull it apart you might be able to see what the hang up is. Possibly bad / worn out bushing or bearing, gummed up internal solenoid lever linkage or sliding gear.
Anatomy of a starter
Anatomy of a starter