Are you using coolant? The tap may 'gunk' up without coolant.
You have Rigid Tapping turned on. Do you have Repeat Rigid Tapping also turned on.
You are going very slow. I tap at 1000 rpm in aluminum and hot rolled steel up to 1/2"-13 and have found the machine maintains better synchronization at this speed. Actually my Super MiniMill will not do it at 100 rpm it just stutters.