What do you tell clients to do if they can’t stay motivated to workout?
You cannot motivate somebody against their will. But you can listen to their answers when you ask them about it.
You are not very specific about the way in which this problem manifest itself. Does your client cancel a lot? Not schedule future sessions? Or not willing to do their ‘homework’? Do they feel pressured by expectations of training?
There are many possible reasons. At some point that client approached you for training and stated the reasons why. You can try to find out what has changed.