Paper Registers on one side - not on the other

I was printing on 80# gloss text ("Coated American Digital Burgo") through our Heidelberg 4 color SM52. Full Color both sides. Sheet-wise.
The first side ran through fine and registered perfectly.
It sat over night and I ran it through the following morning, it was a total of 5 sigs. Each time a new set of plates were loaded I had a hard time getting the back side to register. I ended up splitting the difference between the registration marks to get by.
Any thoughts on this?
Thanks!

 

Register means different things to different people. Are you referring to front marks lining up with back marks? Or are you referring to fit within the images themselves? Or are you talking about a headstop bounce? If you are talking about register marks from front to back lining up with each other then I would ask if you are allowing for "creep" in your prepress dept when doing multi signature work?

 

Probably long forgotten by now but I would say that your paper is attracting moisture overnight and stretching. The cut marks on the back from the day before will be a tiny bit wider than your ones the next day as the paper will be slightly swollen.
Either:
Wrap the paper up overnight.

Back up your sections same day if possible.

Leave the air conditioning on overnight. (If you have the luxury of air conditioning)

Or just split the difference like you are doing.

Obviously you need to change side lays when you turn the sheet over (duh)