Need a little help with my superskimmer 220

[MikeP]

I know it's not the best skimmer out there but I will upgrade in the future. For now it's what I got. It was running fine on my LR tank in the basement but now that I have it on my new display tank upstairs all it does is pump water into the collection cup? The only difference is that it's now in the sump where as before it was set up as HOB. The instructions suck and I just can't figure out why this is happening. Has anyone had similar problems? What can I do to fix it?

Thanks