Expose a sheet of this paper to room light and develop it, that will tell you if it's the paper. Actually, better expose it under the enlarger in steps, but with the head further up, because if it somehow lost sensitivity but not completely, it might still turn an even black with generous room light exposure.
The other possibilities are that somehow it wasn't properly covered/agitated in developer, or the enlarger vignettes at these low magnifications. You could eliminate the latter by printing at this size on a sheet of different/larger paper. But it should also be visible just looking at the projected image.