This is a bug in the Princess ammo selection code; in 0.49.20, she's only able to load ammo from attached trailers. This is, of course, suboptimal.
The bug has been fixed in the development code and nightly builds for quite a while, but unfortunately there is no work-around to get Princess to use ammo weapons in 0.49.20.
On the plus side, it won't be too much longer before the next release!