Filling loop issue; lots of noise then stops filling!

[AMW]

Got a weird issue with the filling loop on my system. Seems to randomly fill fine and other times not, like it’s pushing against something (air?) and then stops filling, makes a load of noise as well. Attached a video to give a better idea!
Is this an air issue? The pipe from the loop appears to go to the bypass return pipe…
Both taps fully open and seem to work fine.

https://flic.kr/p/2o4i8f9

Seem to be getting lots of issues at the moment but slowly getting all the radiators working properly! Quite enjoying learning about it all, and sometimes not when it defeats me, which seems often

 

[AMW]

Any ideas on this? :/ Convinced this is somehow connected to me seemingly chasing air round the system, one day a radiator works fine then the next it can be stone cold. Have to close the return lock shield and draw watery air out the bleed to kind of kick start it in to life. Rarely get air on its own out the radiator, spluttery water coming out is the best way to describe it.
Also 2 other radiators not doing much.

All I’ve done is open the loop to add a bit of water in from the pressure lost from bleeding but seem to end up back at square one. Is it possible the loop could be letting air into the system when it fills or is just a lot of air already in the system stopping the loop from working properly….

Tearing my hair out trying to get this heating system working properly! Frustrating to say the least!