yes, I've been thinking about this. The reason for this is that the spawn location selection algorithm currently used simply selects random locations to spawn. If the location is occupied it will continue to search for other available random locations, but will stop after 10 attempts and will simply place it at the location of the spawner.
If you make a large spawn area, it is possible that random selection of 10 locations will fail to find an open spot.
I am going to be adding the option to specify different algorithms for placing spawns, including things like row-sequential, or progressive-radial. A row-sequential algorithm would be like a simple flood-fill, just selecting successively adjacent locations, so it would guarantee full filling of a spawn area.
I'll add it to the beta_309.zip when it is finished.
(edit)
a new spawn positioning algorithm control feature has been added for the next release. It is in beta_309.zip in the upcoming releases area.
http://xmlspawner.15.forumer.com/index.php?showtopic=53