Whirlpool washer

Whirlpool thin twin washer dryer LTE6234DQ0

recently fitted a new drive coupling now it washes and spins ok but then if I momentarily stop it either with the control knob or open the lid it just wont start again and makes a buzzing noise as if the motors trying but nothing happening if I turn the motor by hand it will sometimes start, I have changed the start capacitor but still get the same results

Check to make sure the coupling halves are not binding on the motor shaft, or the gearcase shaft. When installed properly, the ends of the respective shafts will be flush with the inside surfaces of the coupling halves (in other words, Not extending through them).