It may be partially related, in that I've seen it take weeks to gradually gain a new set of clients after an IP change, which is why I think it's so important to not be abandoning all your clients each time but instead let them update their bridge entries to your new address. If you've been up for 2 months and changed your IP in the middle, you probably cut off and abandoned all your clients after a month just when you were starting to get somewhat known, and had to start over from scratch and are just now beginning to build up a fresh client list again. If you typically get a new IP address every month, you may never be able to build up enough clients to see much traffic with the way things currently seem to work.
And of course there's a large random factor in just which clients you end up being handed out to. If you end up with mostly just people doing a little web email once in a while, they won't add up to much traffic. I like to watch my bridge's status page on globe.torproject.org to see the traffic history and number of connected clients history graphs, and also the Vidalia "Who has used my bridge?" status (or the bridge-stats file in your bridge's data directory) to get more detailed feedback than just the total bandwidth used.
But another issue may be the random luck of the draw of which bridge assignment pool you end up being placed in. As I understand it, to make it harder for threats to find all the bridges and censor them, the bridges are partitioned off into pools which are only assigned to limited subsets of clients via particular distribution methods and client IP address ranges, so that no threat source can find out about bridges outside of the pool they're allowed to pull from. So if your bridge ends up placed in a pool that just doesn't have many clients using it, your info will be handed out that much less often. In the worst case (from the bridge provider's point of view anyway), I believe some bridges are simply held in reserve for emergency use, such as when a common obfuscation plugin becomes censored, so that there's a ready supply of previously unused and therefore uncensored bridges to hand out once Tor figures out how to avoid the new attack method. That's good for the network of course, but I'm afraid it's not very satisfying for the eager bridge provider who's basically left on the bench as a backup in case a first string player gets injured. I suspect there's a lot of churn in that pool as people feel useless and quit bothering to provide the unused bridge. For what it's worth, the globe status page will also show you what pool your bridge has been placed in, which may help reassure (or confirm :( ) that worry.