Help with 2 Radiators Not Heating on Hot Water Boiler System!

@hot_rod - I've never messed with the flow checks before so I'm not sure how to open and don't want to screw anything up. With that being said, there is only one flow check valve for the cast iron radiator zone on the supply side (the other 3 flow checks are for the other 3 zones which each have their own circulator / thermostat control). The circulators are all on the return side for each heating zone and are pulling the water back to the boiler. The picture you screenshotted shows the 3 baseboard heating zones with their own draw off / purge point. The main zone with the cast iron radiators with the largest, white colored ball valve does not have it's own isolated draw off / purge point (it would have to be purged from the blue gate valve on the bottom left below all of the circulators). We do also have a spirovent on the vertical supply line that serves all heating zones. There's currently no air at all in the baseboard zones. Those were purged successfully and since they're in a simple loop we know they leave hot and come back hot so there's no air. I thought for the cast iron radiator zone once most of the air was purged we could bleed all the radiators directly from the radiator bleeder valves (which the techs did before they left the first day).

@EBEBRATT-Ed This afternoon I shut the ball valves on the supply and return side for all heating zones except for the cast iron radiator zone with the 2 problem radiators. I could trace the supply and return pipes (which I believe may go to the problem radiators) all the way to the front of the house in the basement before they disappeared in a wall (the problem radiators are on the 2nd floor above the area where I was in the basement). The supply and return pipes felt very hot where they were accessible for me to touch them. Oddly enough it seems both problem radiators are not getting heat from the supply valve side. They seem to be getting heat from the return side (see first 2 pictures below from my IR camera - excuse the picture of a picture, but I can't find my cord to upload the pics directly from the IR camera). The radiator that didn't really heat much at all seemed to get a tiny bit of heat now (99º with IR gun) where before I felt almost no warmth at all (83º with IR gun the other day - although the supply valve the other day got up to 99º). It seems the flow has reversed for that radiator since the first problem day ( it's heating now from the return side instead of supply side). I had the heat run for about 45-60 minutes this afternoon. The rest of the working cast iron radiators got up to 145º-155º and were heating well (see 3rd IR picture to see a working cast iron radiator fully heating).

@pecmsg - I tried bleeding the individual problem radiators from the bleeder valves again while the heat was running. No air came out - only cool water. I also tried opening and closing the ball valves for the supply and return on the cast iron radiator heating zone while the circulator for that zone was running - I don't think that did anything.

As of now the heating company is planning to come back next week to replace the supply valves on the problem radiators. I really hope that fixes them, but if anyone else has other ideas or thoughts on this, please let me know.