Contrary to what you may think, babies don’t know how to drink out of a bottle. Especially if they were purely breastfed. They don’t like the taste of formula, nor the feeling of a plastic nipple, and it takes some time to get them used to it.
my advice
If your baby is only on breast milk, introduce bottles using mother’s pumped milk very early on (within 1st month) - this helps introduce the concept of eating out of a bottle. It also helps dad learn how to feed. Its a two way learning! Some say trying different bottles helps, I think that’s a luxury, and a baby eventually learns to drink out of whatever bottle you give it.
Give yourself plenty of time to transition to formula. You may fail once or twice. This is especially important if mom is ending her maternity leave and going back to work, and dad is helping take over.
Two strategies can work - the first one worked easier for us.
Cold turkey - Decide on a weekend to transition her to formula. Nothing else is planned for this weekend. Start first feed with mom. At the next feed’s time, only have formula available. There will be resistance. If the baby pushes it away, take a break and try again in 15 minutes. Eventually they will start drinking the formula out of a bottle.
Transitioning - start with full bottles of breast milk. Through the week transition to 3/4 breast milk 1/4 formula. After a couple of days switch to 1/2 breast milk 1/2 formula. Towards end of the week go 1/4 breast milk and 3/4 formula. Beginning of the following week go to 100% formula.