Your baby has become used to synthetic nipples. May be its taking him extra effort to get milk from your nipple. You need to have patience and try to get him used to having your milk.

As I can understand he has not had bottled milk for long. You can change his habit is short time. Start with one feed of your milk and slowly increase it.

Its just that you baby is finding it harder to suck milk from your nipple. You have to keep offering him breast milk when he is very hungry he will get onto it. Maybe early in the morning you offer him breast milk. Slowly he will get used to once again.

Definitely your baby is used to the feel of the synthetic nipple. You have to get her used to your feeds. Another thing that might be a problem is that she might be used to lying on her bed and having milk from the bottle. If you are trying to feed her taking in your lap she might be finding the position uncomfortable.
Try to put her in a comfortable position while feeding.

Your baby has gotten used to sucking on synthetic nipples and she it finding it and easy job than to suck and get the breastmilk or may be is has started liking the taste of formula.

First try pumping your milk and giving it to her in bottle. If you see that she is ok with the taste then as suggested by other get her onto your milk slowly. Start from once in a day and slowly go on increasing the number of feed by you. When you start off offer her breast milk when she is really very hungry.

I think it just needs persistence. Just keep him offering your milk whenever he is hungry slowly he will get used to having ur milk.