Can't run skimmer right after a water change

[DanG]

Is that normal?
It's a prizim HOB skimmer with the deluxe upgrade(skimming basked and media inster), I've also done the mod by adding a 2 inch section or riser tube.
I find that if I run the pump right after a 5 gallon (out of 40) change, the collector cup just fills with water and no skimmate. It usually takes a day or so to be able run properly again.