Low boiler pressure valiant ecomax 830?

The pressure on my boiler is at 0.75 which is below the green bar so I have been trying to raise the pressure. My heating is obviously off at the moment so this might be affecting it.

I have turned the black knobs on either side of the heating loop but it doesn't sound as if any water has come through. When I have done this previously the sound of the water running was quite apparent but I can't make it work now.

Any idea what I might be doing wrong?