This often occurs because you have a full doff roster. Active duty doffs do not count towards your reserve roster.
True. Unless you have bought doff roster slots in the C-store, you can have a total of 100 doffs in your roster with 5 on active space and 5 on active ground for a total of 110. If you have 100 doffs in your roster you won't be able to remove any from active duty until you either put another on active duty, put 1 in exchange or mail, or dismiss 1. Even if you have none on active duty, if your roster is at 100 you won't be able to get any more until you do 1 of those things.