I used to think that skimmate had to be black and gunky for the skimmer to be doing it's job. An LFS owner showed me a handout from a skimmer manufacturer, and that changed my mind.
The handout noted that skimmate should look a bit like tea or gingerale. The LFS owner made the point that it takes more time to get dry gunky skimmate, and if you dilute it with clean water, you pretty much break even as far as what is getting removed from the tank water over any given period of time. Wet skimmate will apparently remove more than dry skimate in a shorter period of time.
He also mentioned that dry skimmate builds up around the collection cup, and bubbles will burst on the build-up, leaving all the crud to fall back into the water.
I don't know if any of this pertains to your situation, but hopefully it'll be of some use.
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Cumshot mouth