Fussy while breastfeeding

Beebee

My 5 month has episodes where she quietly nurses and content. In evenings she can nurse for an hour and gets fussy. Flares her hands and feet, cries and searches for the boob. Nibbles than cries again. I walk around, burp her, talk to her but she's still fussy. Her eyes get tired and again googly. There's plenty of milk so it's not that. Not sure how to fix the problem?