Do you have CPsec enabled on your controller?
If you do, the RAPs will be assigned the sys-ap-role
If you do not, the RAPs will be assigned the ap-role
If you have it enabled, the VRD says you cannot assign a pool to the sys-ap-role. In this case, make sure the RAP pool is listed first in your listing of L2TP pools; even if you have to delete them to make it first in the list. If memory serves me right, the RAPs should pick from the top pool in this case until it is depleted.