I have a problem upshifting. It will upshift to the the third smallest gear but after that it gets stuck. The funny thing is, I can't tell why. The derailleur has a full range of motion, and I have adjusted it properly. The jockey (lead) gear on the derailler positions the chain over the correct gear, but the chain doesn't want to move off the current gear and onto the next one.

The thing obviously wants to shift, and it makes that rattling noise that sounds like the chain is about to move to the next gear, but it just cant for some reason. The derailleur is pulling all it can to get it over, but the damn thing wont move. The chain is pulled by the derailleur over the right gear, but its like some invisible force is making the chain cling to the lower gears. It just doesn't make any sense.

What's going on?