Thank you for your question, I wanted to know the reason of starting formula: is it you have to join back or your milk is not sufficient.
For breastfed babies they gets habituated to mothers milk which is just appropriate for that baby and then taste wise they don’t like that formula and also there can be nipple confusion, as baby is spitting more may be baby is not digesting that properly
Next Steps
Start with very small amount
Online consultation if further advice required
Health Tips
You can try with katori spoon or palady