My iPod won't back up to iCloud?

My iPod 4G backs up to my computer, but whenver I click the button back up to iCloud it starts, but then never finishes. Everytime I press back up to iCloud a message pops up that one of my apps couldn't be transfered because I bought it on my iPod on a different iTunes account (not the one I usually use). I click okay but whenever I click okay the iPod doesn't back up. I haven't backed up my iPod to iCloud in 4 months, so I really want to back it up, but it won't let me. There isn't a message that said backing it up was unsuccessful and I'm sure i have enough room too. It just stops backing up whenever i get the message that one of my apps didn't transfer.