Firstly, my advice for painting yellow.
Don’t use a black or white primer, use brown.
Yellow works really well over a brown primer, you’ll see a massive difference!

I’ve found with most airbrushing problems there are two solutions:
1) The brush needs cleaned more thoroughly
2) Adjust the mix of the paint.

1) What are you cleaning the brush with? If it’s just water you need to use some form of airbrush cleaner.
I use two different types, one is vallejo airbrush cleaner (looks a lot like water), which is my main cleaner, in addition to this I use a solvent based airbrush cleaner that comes in a spray can. This is for the more stubborn to reach areas and only gets used on “deep” cleans.
Sometimes your brush will need to be “totally” disassembled (not to the point where every O ring is removed, use common sense when taking it apart for cleaning), but certinaly more than just taking the back off and removing the needle.
paint can get in the places you least expect it. ESPECIALLY if you’ve had a blockage at some point, as the paint has to go somewhere! so it’ll usually end up somewhere it shouldn’t inside the brush.

Once you’re sure it’s 100% clean…
2) The mix of paint,
this is a really tricky one to diagnose and fix online, as there are lots and lots of things that can go wrong here.
the standard answer is to thin your paints to the consistency of semi skimmed milk,
if you do that, there’s less chance anything can go wrong, but it’s still quite vague.

Do you mix your paint in the brush? or seperate? I tend to mix mine in shot glasses, then pour it into the brush as I need it. I also don’t overfill the cup on the brush, if there is a blockage, I want the minimum in there as possible.
If you mix in the bush, there’s a good chance you’ve forced a lump of the thickest stuff way down into the brush first before your thinned stuff, giving you an inconsistent mix (overly thinned at the top, not thinned enough at the bottom), and exponentially increased your chance of a blockage right from the word go.

Is there any reason why you use the citadel stuff? I’ve not heard great things about the citadel air range, but honestly never tried it myself.
My collection of paints is a mix of old GW, vallejo & Army painter.
By far my . favourite is vallejo, I have very little issues with vallejo and the range of colours available is astounding, no matter the colour there will be a vallej equivalent!

Paint not adhering tends to be a dirty model, usually mold release agent still on the figure.
Best course of action is to use warm soapy water to wash the model before priming.

Hope this helps.
Thanks

Chris