I have been playing hardcoe lately and I have been relying on Veronica. I just stepped out of Vault 34 and she refuses to regenerate health. I have to baby sit her now and force-feed her stimpaks. I tried dismissing her but she still doesn't heal health
Try taking her into battle with a few easy enemies (Mole Rats, regular Geckos and such) and let her fight them. She should fully heal after you finish.