I was finding that when I took my dérailleur apart for cleaning, the jockey wheels in the cage always had a small amount of grit in the sleeve bearings, causing them to bind slightly.

I have overcome this problem by using oil in the bearing sleeve, but before re-assembling the small metal cups on either side, I fill the cups with waterproof grease.

This effectively seals out any contaminants and even after riding on a dusty road the jockey wheels still spin smoothly.

Regards,
Roger