Neil, that is the most common malady for new divers. Though you said you have no problem equalizing you are probably equalizing after you feel the need.
So, imagine this scenario. You are descending. You go down a few feet. You feel the discomfort and equalize and feel fine and continue on and repeat. But during the brief moment before you reached equilibrium you subjected the middle ear to a negative pressure environment. That relative vacuum pulls a tiny amount of body fluid from the surrounding tissues so as to create an equalibrium.
Then later during the dive you may slowly descend several feet as you are cruising along but don't really notice because you are a careful diver and don't make sudden depth changes. The slowness of the change masks the pressure differential sensation and you don't feel the need to do a equalization but more fluids are seeping in.
It may take a day or two days to reabsorb the fluids but you are on a dive vaction and not only do you have more dives that day you have more days that week.
So, you are subjecting your ears to repeated minor middle ear trauma without the opportunity to recuperate. The worst case is the accummulated fluids create a nice petrie dish for a middle ear infection.
Additionally, the Valsava, the most easily and commonly used equalization maneuver is crude and abrupt and it is easy to overdo it if not careful. So, you can be slamming your ears from one pressure extreme to the other. Look at the delicate structures in there that are getting pounded.
Some things that may help are to try some other equalization techniques and to "pre-pressurize" to avoid the state that encourages fluid accumulation.
Check out this site for some great info that helped me:
Prevention of Middle Ear Barotrauma and a
video by the same physician, Edmond Kay, M.D., clinical assistant professor, Family Medicine, University of Washington School of Medicine; diving medical officer, Dive Safety Program, University of Washington
