Flat Beer!!

hey guys, so i had four kits of beer going recently, one light, one brown and two stouts. I did them all the same way, about 7 too 10 days in the pail, about 16 or 17 days in the carboy, then racked it too a pail, added a cup and a half of sugar that i boiled in a cup or two of water, stirred it and bottled it, i let it sit for about two weeks in the bottle at room tempature, then refrigerated a bunch of each, its almost seeming like the darker the beer the less carbonated it is. The light beer isnt bad, same as the brown, however the two stouts seem pretty flat. Should they be done differently? what could be some reasons for them being not so carbonated. Thanks guys!

They might just need more time, give all the bottels a quick shake and let them sit at room temp for another week or two.