Over the weekend I tweaked the server settings a bit. It seemed to me that during January, when the server scheduler was failing and only the Axis server was running, more players showed up. My theory is that since the server NAME was changing when it changed from Allies to Axis and back again, this made the server less likely to appear on server browsers. Based on what I can see from server traffic stats, traffic went up starting early in January and dropped again to pre-January levels after I fixed the scheduler.
To test this, I changed the server settings so that the server name no longer changes. Now, it's always "Tanks'nPlanes Unlimited CO-OP". I also changed the intro messages and the timed announcements so that important information appears in capital letters. Apparently this attracts players' attention a bit better.
Anyway, I'll monitor things over the next few weeks and see what happens.
Next I plan to shuffle the maplist a bit to put the more popular maps at the top and group the Road To Rome and Secret Weapons maps.