I too have a 6100 printer, so I'll take a stab at helping. Let me say first, I don't print a lot, and the 6100 seems to have a mind of its own a lot of the time. Everything has to be just right and your tongue has to be in the correct side of your mouth.
Anyway, make sure you use the page set-up button on the left panel of LR first to get all that correct. Also, maybe even before that button, go to the printer itself and the load paper button sequence, and scroll through those choices to get the paper size you need. It may be listed as an A or B something-or-other (like 13x19 is A3+). I think somewhere in there is your problem. Good luck.