I would think that more flow from the indoor boiler through the flat plate while heating it less on the first pass would make the loop the second time before it goes out to the zones. It's a really short loop, much shorter than the zones. So more flow would be better with more temp gain. Thoughts?
You might have an emitter issue though. They're probably sized for 180+,℉, adding a few would make a huge difference, otherwise slinging more water doesn't help much. Cleaning lint off could help a bit too.