The starter solenoid could be hanging up.
On my L2500, when the solenoid is energized, there is a very audible clunk as it throws the pinion back to engage the flywheel. If you don't hear that clunk, solenoid might be sticking.
I use my tractor infrequently, and the old battery typically gets weak between uses.
If I twist the key and I get that clunk, but it doesn't crank, I know the battery needs to be charged.
If the solenoid is sticking, a firm tap on the starter mounting bosses with a hammer might free it up.
-Paul