I'm sure you are familiar with the little audible click from the muting relay after lowering the arm.
Can you hear this click in the cases when it does not lower ?
If the click is heard - repair the lift/lower mechanics (solenoid etc.)
If the click is not heard - the tonearm is not exactly above the lead-in groove and the threaded shaft will
need cleaning/lubricating and the servo motor belt renewal.
If could also be a capacitor problem, that would be no surprise. A kit is available for this but check first for the relay click.
Martin