The amount of cosmid DNA prepared is modest, but is more than enough to make an M13 library.
Antibiotics decompose with time.
First thing in your working day , look at your plates. You want small colonies. Usually, these are apparent after overnight incubation. If you see all large colonies, proceed with caution, as you may have E. coli containing a cosmid with deletions. Transfer the plates to room temperature, so that the E. coli does not overgrow.