As I said, I don't have any of the recent Citadel paints for that exact reason, never mind the reduction in quality / size of pot / price hikes... they have a reputation for being poorer quality and drying too fast so I never bother with them. Plus most of my original Cidadel colours from 20 yrs ago are still fine and VMA & Coat D'Arms filled any other requirements.
( I do wish I could get hold of some Moody Blue .... oh well.)
If you need certain colours, try looking up Coat D'Arms, these are the "old" citadel paints as originally made by HMG of Manchester. I have several and they work fine and are colour compatible with the second incarnation of Citadel.
Coat D'Arms are the round 18ml pots with snap lids, and pretty competitive price of ~£2.20.
My recommendation is not to use retarder, cut it 50% with Vallejo airbrush thinner which you can get a small bottle of to try for less than £3 ish.