Baby doesn’t want formula

Since my girl has been born she has been slow to gain weight so I breastfeed and offer formula after every feed too. Sometimes she takes only 5ml of the formula, sometimes she has lots! Recently she’s been rejecting the formula completely. We finish on the breast and then when I put the teat in her mouth to offer the formula she does a face of pure disgust. Sometimes she’ll gag after only a few drops have entered her mouth. She then just pushes the bottle away and won’t take it. I don’t think it’s due to her being full as we do one feed a day that is just formula (often the last feed of the day when my supply is low) and she’ll often do the same thing! I know the breast milk and the formula will have a different taste and consistency, but I really need her to be happy to take the formula too to make sure she’s putting on enough weight! Has anyone else experienced similar? I don’t really want to change the formula as it’s worked well for us so far - no tummy, gas, wind, nappy issues etc!