Envelope Printing on HP 5610

I am having trouble printing envelopes. I have a HP5610 all-in-one.
I put the envelope where I am supposed on the right hand side of the printer, but when I hit the print button the cartridges slide over to the left side and print on nothing. The envelope is on the right side of the machine and comes out blank. If I put the envelope on the left side , the envelope has a problem loading. It worked fine until a couple of weeks ago.