Nipple Shields

So I made the decision two weeks after I had my baby to try and BF because I got frustrated with all the issues I’m having with his formula, and went to Walmart and bought some nipple shields and he latched GREAT the first time we tried it. However, sometimes he latches great and stays on the breast for awhile then others it’s like he’ll latch, eat for a few minutes, then completely unlatches and he can’t figure out how to latch back if that makes sense? It’s really frustrating for us both and then I wind up giving him a bottle because he gets so upset.. any advice?