Stephen, here are a couple of thoughts. The running wheels may be worn out. There is usually a specialty window and door hardware shop around that may be able to supply replacement parts. You can usually take the door out to check this by lifting the door up and sliding out the bottom. Take the old hardware off, if it looks shot and bring it with you to the door hardware shop. WD40 is a great product, useful for many things, but not the best for long term lubrication in a wet environment. I use a product called Super Lube, a synthetic lube with Teflon (tm) in a spray can, but there are many similar products. Just check the aisles at a mass merchandiser, auto supply, or similar store. White moly grease, used sparingly, is also an answer but tends to pick up sand, dirt and other grit if you use too much.