Carbonation issue

[Sokratees9]

Just wondering if anyone can help me out with a carbonation issue in my kegerator.

I brewed a Belgian Dubbel about a month ago, hooked it up to 40 psi for a day or so, then brought it down to about 20 or so for a few days longer, then I finally brought it down to 15 for serving.  All was good, it foamed up quite a bit, but I'm ok with that, as the head was tight and the beer tasted great, and I just needed a little patience.

A couple of weeks later I brewed another beer, since my other keg was empty.  I took this opportunity to disconnect all the lines, and give them a cleaning, which I hadn't done yet (first set this up in October, so thought it was definitly time to do it).  So I ran PBW through the beer lines for about 10 minutes each, then rinsed with water and hooked the taps back up again.

And since then the Belgian Dubbel has been pouring quite flat.  A head does form, but it dies down quickly, and sounds like coke fizziing out when poured.  The beer tastes good enough, but not as good as before.  On the other tap, it seems fine, so I actually pour in a bit of that beer (about 20%) to give the beer the life it deserves.

I've no idea why this would happen after cleaning.  Could it just be that the beer needs more CO2?  I've upped the psi on that keg to 22, and will see if that helps, but I haven't noticed a difference yet.  I'll give it a couple of days, as I reckon that will be needed for carbonation to push all the way to the bottom where the beer is drawn from.

On the Belgian Dubbel, I have this type of ball lock out connector:

https://www.themaltmiller.co.uk/product/stainless-steel-disconnect-with-flow-control/

On the other keg, it is a normal one (with a flow control tap).  I might swap them over to see if there is any difference, but I can't see why there would be.

Any thoughts?  Could it simply be that my initial over-carbonation was right, and the carbonation levels have got too low over the course of a few weeks, and I'm conflating it with the line cleaning.  I don't see how/why the line cleaning would have affected it so much (or seemed to do anyway).

Thanks for reading a looong post.