So I have a Composite duplex with 7 stops. The up calls are on one car and the down calls are on the other. Intermittently, one car will park at the 4th floor and refuse to answer a down call at that floor. It will just sit their with its doors shut. The building typically cycles the main line and the car is usually in service again by the time I arrive. I have noticed that when the car has a car call in the down direction it does not stop for down hall calls as it passes them.

Check your DC relay contacts.

DC does need to pulse to start the slowdown but there are some n/c contacts in that circuit on the group panel logic door in that circuit you can check or replace. Wish I had a print.