It should take thee things to energize the solenoid: the key switch being ON, the Forward/ Reverse microswitch being activated (a direction selected), and the accelerator switch being activated. If all three of those things are happening, the solenoid should stay energized while the engine is running. So, it’s not quite that easy to see what is wrong, but probably the easiest place to start is at the solenoid. When energized, on the small contacts, one of them should show the battery voltage (around 12 volts), and the other should show 0 volts. If that is happening, then the solenoid must be bad. If that isn’t happening, you need to determine which switch is preventing it from happening. Good luck, Ron.