If you use 2.55 ET, then there can be a problem if you have to many map.pk3's in your ETMain directory, that is/was a bug wellknown

But i would recommend using a objective rotation, that way the server only count 1 map at the time, and you dont have problems with making that rotatioas large as you wants.

And remember to use the correct mapnames when you makes rotation. and the correct name is located in the map.pk3 file in a directoryalled /maps (taken from top of my head ) and is named "mapname.bsp" just used the "mapname" part in your rotation.

When i started building ET Servers a "few days ago ", i just used the pk3 name in my rotation/campaign and i had lots of problems until a nice friendly person told me where to look for correct name, rotation/campaign wont work unless maps are properly named.