Flare them to other bettas, mirrors, etc. Make them expand/spread their fins. This can be done between 5 - 15 minutes after feeding.... they should poop. Then siphon their poop and uneaten food.
I usually flare them for a much longer time. I sometimes flare them for an hour or more. I don't totally open their dividers. Just lift it a bit, about 1". OR I use a divider with a small hole in the middle (diameter of a pencil). They won't flare the whole time, but they will move a lot. Use different methods so they won't immediately know where to find rivals..... oh when using partial divider method, do not open the top part because bettas surface to get air thus will always see each other.