[Mike51Merc]

Check for voltage drop at the solenoid. These solenoids are sensitive to voltage drops and will be slow to engage (or not at all) if voltage drops below 6 volts. Compare voltage at the battery with voltage at the solenoid connections.

I had a similar problem, the solenoid put a draw on current but wouldn't pop. It often happened at night until I figured out that the combined draw with the headlights made the voltage drop worse.

If you find a drop, then clean all connections on the solenoid harness. In my case I had to rewire the solenoid feed line and I used a heavier gauge wire.