The advice on avoiding WD-40 as a lube is spot on. It's ok for displacing water or used with something like a scothbrite pad for removing surface rust, but don't use it as a lubricant. I would take the wheel set apart; you'll have to remove the corner guard, probably by accessing the screws under the bag liner. You may find the wheel is just jammed with a piece of string; or as I once found, about an 18" long piece of dental floss that was caught in there (no idea how or where my bag picked up floss)

You may or may not be able to remove the bearings. If you can, soak them in WD-40 or paint on some naval jelly if you see any rust. When they are clean and wiped free of WD-40 or the jelly, I would lightly lube them with something like 3-in-1 oil. Or Breakfree CLP which is used in firearms. It's a great lube and doesn't attract dust or dirt. You can get small bottles in sporting goods stores, some hardwares, and on Amazon.

http://www.break-free.com/?location=/products/index.asp