When a contributor stops contributing
If you plan to change anything in your authorship agreement, you should bring this issue up as soon as possible.

You could politely ask if the person is still interested in working on the paper,  e. g.  ask "I've noticed that your responses for this paper come slowly. Probably you are busy with other things. To complete the paper I would need... from your side. I could also try to finish the paper without you, but wonder if you still want to be a coauthor." (Just a first shot for a formulation...)

You don't know the motivation on the other side, so it may well be that the coauthor is really busy, but feels obliged to continue to work on the paper and they would be happy to say "I'm out." I have they can't work on the paper but insists to be coauthor, that is another topic for which your field plays a large role.