Good-o - I can talk about enamels; acrylics I've not yet mastered in the airbrush...
It's possible the mix wasn't quite thin enough and that particles of paint simply dried in the internals requiring a full strip down, or maybe a build-up of deposits (which you thought had been properly cleaned out) which finally tipped the balance. I'm naughty and mix in the cup as I go, but if you're not careful and haven't stirred the paint sufficiently it can bung up the brush. I warm enamel tinlets on a radiator for about 10 minutes before shaking and stirring which helps to break down the thick gloop of pigment.
I always blow through with cellulose thinners between colours until it flows clear, and then squirt through with Liquid Reamer which is a xylene/acetone solvent, and at which point you find more colour flowing out! It's incredibly dangerous stuff if breathed in, but will shift dried deposits from the internals where cellulose fails. That should be enough for a colour change, but isn't enough if you want the airbrush to work without fail next time.
After an airbrushing session I always run through with cellulose until clear, run through with Liquid Reamer, strip the brush and clean the parts with xylene on some kitchen towel or using these brushes, lubricate if necessary and reassemble so next time it's ready to go. I think the first few times I stripped my Iwata it took nearly an hour, but I've become so familiar with it, and which parts need most attention, that I'm down to about 15 minutes or so.