Okay, we are getting somewhere. With flow out of your heater then its says your problem is in the green arrow return lines.
The problem is there is no easy way to get into those lines short of moving your heater so you can put a hose with water into the return pipe and seeing if you have flow out your returns. My bet is you will not. You have some blockage. Maybe the valves in those four lines into the ground are not properly opening when you turn the handle.
There is a point where you need to punt and bring in a professional and this may be it. All I can suggest is you really carefully examine the return valves and double check there are no plugs in your return lines.
It may take someone cutting the pipes open to check each line and then redoing the plumbing. PVC is cheap. It is just the labor time to figure out what is going on in those lines.