If you have a pasture to put them in I would lean towards keep them.   The cost to keep them won't be much different than if you started over with new heifers that don't have any birthing/mothering experience.

I could be wrong but if they all calved once before and the bull was the reason they didn't breed, I'd think that the risk of them not breeding (of their own fault) after having a full year to recuperate should be quite low.