When you filled the spa, how did you do it? Did you just put the hose in the tub and let it fill? It is possible that you have an air bubble in the line.
Open up the skimmer, find the filter with the gray cap and stand pipe. Put your hose down this pipe and turn on the water. Try to get the water to force down the pipe. If you have a bubble, this will push it out.
Not sure if that is the problem, but it might be worth a try.