my baby is 4 month old he is not having milk from by one breast what could be the reason behind this

A. Aversion from breast is common to be seen and there is nothing wrong with it if your child is not taking feed from one side then you can express the breastfeed and give it with the help of a feeding bottle as this way you should feed from both the sides otherwise the other side will stop making milk plus the breast size will have a permanent difference in size