sleepy eaters

my twins seem to be sleepy when breast feeding but do better with bottle feeding. I am having issues with a low supply but I can spend a half hr with them and they still eat a ton after when I have to supplement with bottle or BM or formula. I can see them suck and swallow here and there but they are lazy about it. Should I spend so much time hoping they fill up even though they aren't or just give them 15-20mins and go to bottle so I know they are filling up. I like to put them to breast as much as I can to help my supply increase but nothing seems to be helping that matter. Any tips on what to do to get them more alert or if I should keep a time cap on nursing them? TIA