Manifold flow indicators not working

[Triassic]

I’ve recently commissioned my UFH and two of the flow indicators aren’t working. In the photo below you can see the left hand indicator has no water in it and the flow and return pipework is cold, even with the valve fully open. Has anyone got a cross sectional diagram of these valves and flow indicators, as I need to strip them and get them working.

 

[JFDIY]

It'll be air locked.

 

You can try closing off all the other loops to force water into them one at a time, but I've read it on here that you feed the hot leg of the manifold with mains pressure via a hosepipe, then open each loop in turn to bleed the loops through fully.

[Nickfromwales]

If these haven’t been blown through, aka ‘purged ‘ with cold mains, then this will be a non-starter. You can run the manifold pump for a month of Sunday’s and it’ll do zilch as even the max flow rates are just too low. 
There will be zero need to strip a brand new manifold down. Don’t do that. 
 

[Triassic]

Thanks for the responses, I’ll try the mains purge approach and see if that works.

[Triassic]

I’ve purges all the loops and they are all fine. I then applied pressure to the hot manifold and got water out of all but two of the points. Having removed the red collar from round the flow indicator I found that if you unscrewed the plastic flow indicator about 1/2 a term the port opened and water began to flow..