inconsistent margin placement

[sorbiegunner]

I am printing out greeting cards on 125x250mm card blanks on my Epson ET-2810 printer. There should be 3mm margins all round but the front and back margins are different every time. The design is set so that there should also be a 3mm margin either side of the fold but the image on the front page always has too big a margin at the front and prints up to or over the fold. The layout all looks correct in the print preview but it doesn't print like it shows in the preview. Is there anything I can do to get this to print correctly consistently.
I am using Affinity Publisher 2. I can't compensate for this in the design because the margin is never the same twice it moves by as much as 5mm forward or back. It is printing straight and the long side margins are spot on.