Constipation Issues

Hey guys I’m looking for some advice my LO is 8 months old and still struggling with constipation issues, when she was 6 months we switched from enfamil gentlease to nutramigen hypoallergenic formula, she only poops once a day (has been that way since birth) but some times it’s a normal bowl movement, others it’s the hard rabbit pellets or hard balls, she doesn’t seem to be in any discomfort when she goes but i know she is constipated, she’s been refusing a bottle the last couple of days, she normally takes 6oz every 3-4 hours now she hardly drinks 3 Oz, and we had a vomiting episode over the weekend with no fever, the only think i can think causing this is her being backed up. I’ve tried the wellements baby move,been giving that to her twice a day for two weeks now it doesn’t seem to be improving. I’ve read to try MiraLax, any advise or opinions greatly appreciated i want to help my baby out!