it happens to me as well.. and only on a couple of listings generally at a time

the gallery picture is there, and then when it auto-relists the gallery picture is gone but when i open the listing all the pictures are there

It tells me theres a problem with the gallery picture and click here to fix but nothing fixes it

all i can do is either remove all pictures from my listing and then re-add them all, or let it run its course without a gallery pic

Another ebay seller i work with also has the same problem