Welcome to CT!
Did the vet give the heifer any antibiotics after he pulled the dead calf? Sounds like she probably has an infection, especially with a dead calf in her for that length of time. I would suggest doing your best to get her caught again, even if it means bringing in 20 of her closest friends (at which point you sort her out). I'm a firm believer in Vit B Complex for sluggish calves and it may help your heifer, but I suspect there's a lot more going on and she'll need a lot more than that. What is your location? Do you have access to a chute, headgate or some way to restrain her? Once you get her in, can you take her temperature? Sooooo many variables!