Help With Cold Radiator Problem

[Lawrie0523]

Replaced a few radiators and am having trouble now that I have refilled the system.

The last two radiators on the loop (downstairs) are not getting hot. And I can't fathom it out, full description of what I did and what I've tried follow:

1. Drained system (expansion tank fed)
2. Replaced 4 rads various locations.
3. Refilled system and added Sentinel X800 Cleaner
4. Ran boiler for about 2 hours.
5. Drained again and let it flush through until outflow pretty much clear.
6. Refilled in order as follows; upstairs rads off, bleed downstairs 1 by 1, open upstairs and bleed, bleed valve next to pump. Also added Sentinel x100 inhibitor.
7. Switched boiler back on. All rads hot except last two.

I tried switching all the others off (at least all that I can a couple have seized valves) but that hasn't solved it, also cranked the pump up full but no change. Here's the bit that is really puzzling me. The last rad has a drain point and if I open it the water flows through and the radiator gets hot but as soon as I close it back off it goes cold again. The one before also gets hot if I bleed it for long enough but again it goes back to cold afterwards.

One is new and the other is old, neither has a TRV any suggestions?

Thanks

 

[MeldrewsMate]

Last rads on system? Do you have a single pipe system? If not all rads are parallel loads on the pump, so try turning all other rads off to see if the two cold rads become hot.
Is there a possibility you have connected these two rads incorrectly, by supplying each end with the flow or return?

 

[Lawrie0523]

Tried turning everything I can off, couple of the other rads valves have seized so can't shut them off but I reduced the load from 9 rads down to 3, but it didn't help, pretty sure they are all parallel. The new rads were straight swap out of old ones I haven't changed or installed any pipes.