Hi Mark,
Welcome to the forum.
Is this your first go at a pusher mission?
If it isn't then you will already know all that I'm about to say, and, sorry for stating the obvious.
Firstly, have a very good look around and make sure that you have not missed any mooring ropes between the barge and the dock.
Next, are you using the proper connections between the pusher and the barge?
In the attached picture, if you look at the fourth icon from the left at the bottom of the screen, you will see a stack, the icon you need is the bottom one.
Lastly, if you use the search function then you will find quite a lot of comments about rope stretch that might help you.
Regards, Dave