I have the interesting situation at my dojos in that I'm alittle like the spare wheel. As our arts require partners to act as tori and uke, I don't have a regular/grading partner. There have been a few uni students who have joined us who have come as a group so stick together; there is also a couple of teenagers who've joined and a couple of guys I've graded with before who've either left or don't attend regularly anymore. It can be frustrating, especially when coming up to a grading. I've been doing well just recently as there has been another brown belt whose regular training partner hasn't been around and I've managed to train with him, but there's no certainty we'll train together regularly in the future.
That's interesting.. With the exception of Calgary (Aikido Bozankan) in the mornings, and Aikikai Hombu in Tokyo, all the dojo I've practiced in have switched partners quite regularly during practice.
Does your dojo not encourage practicing with as many different people as possible? After all, one needs to be able to "aikido" anyone, not just a regular partner..