I'd have to agree with Mona. My opinion is that, at least in our area, we have had series upon series of high winds or cooler temperatures and rain to keep them roosting. ALthough I have 13 cats now, none came from my garden with over 75 milkweeds. I have onle seen one Monarch so far.
I have several photos from last year showing Monarchs and bees of various species on the same purple coneflower.
Be patient, when they get the weather, they'll be there in great numbers.