If your breasts are engorged in between feeding and expressing milk, then you most likely still have adequate milk supply. Maybe it's a pump problem. If on the other hand, your milk supply is actually decreasing, then there are many things you can do to increase it again.
This is a link to my blog:
http://pedramusings.blogspot.sg/2015/06/increasing-your-milk-supply.html?m=1