mystery ceiling joists are driving me mad
"Screws in the drywall" WILL NOT hold a 25lb. fixture!! Are you sure which direction the ceiling joists are running?? Normally they run the shortest distance of the room, sometimes they change directions in a portion of the room and are supported with a "bond" timber (beam). Plaster ceilings perhaps with a layer of drywall over them (you said an old house)?? Stud finder may not work, nail may not be long enough to hit the joist.....
An existing ceiling box may have been "spanned" between two joists with metal brackets to center it in the room, so if you're trying to "guesstimate" your 16" or 24" centers from that it may throw you off. Hard to say.
I'm having a bit of a problem following too. How are you "seeing nails from the wall studs that extended into the ceiling"?
A picture might help......
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