I am not an ML10 owner so may be completely wrong, but, my understanding is that there are 2 levers to engage the self act. One is the conventional half nut engagement lever on the apron and the other is at the headstock end to engage drive to the leadscrew via a sliding sleeve over the split leadscrew with a slot which engages with a stud on the business end of the leadscrew.
So, the question is which lever is the OP using to engage the self act. If it is the sliding sleeve lever and the sleeve is not free to slide then the symptoms described will result. The solution may be as simple as a squirt of oil down the sliding sleeve.
As I said I don't own and have never operated a ML10 so I may be talking rubbish, if so be gentle with me I'm only trying to help.
Jim
Edited By Jim Nic on 12/10/2020 09:45:08
Edited By Jim Nic on 12/10/2020 09:45:44