Following the NNC, after you position the flap level to one lesser flap, call the cabin and ask for a coffee! Don’t expect the flap to move immediately. Order the coffee, hold the NNC, carry out the missed approach, pick up a holding pattern, wait 2-3 minutes then you should see the flap retract back a step up and eliminated the flap split problem. Then come back try again after the coffee.
The gist is give it time.