6 month old and formula

Lately it seems like such a struggle to get my LO to finish a bottle in one setting. He’ll give hunger cues and I’ll go to feed him and he barely has an ounce before getting annoyed and not wanting anymore. Sometimes it takes two hours to finish one bottle. Or we’ll waste cause he doesn’t finish by the time of his next nap and I can’t save it.

I offer him 4oz bottles. His pediatrician said at his age he should eat around 4-6 ounces every 4-6 hours. At minimum that’s 24 ounces for the day. Sometimes I don’t think we reach that.

I know baby will eat when he’s hungry.. but is it possible he isn’t getting as much as he should? We feed him solids (purées) 3x a day.. but always give formula first.

Just wondering if anyone else experienced this. I’ve tried the next flow nipple and it doesn’t seem to help. Not sure!