Water Injection question

[mazspeed]

I have a cooling mist unit in the trunk of my car to cool off the supercharger intake air. I was told that every 2 tankfulls I should be empty on a 1 and 1/2 gallon water tank. 20 tankfulls later, it's only 1/4 used. Is it not working correctly? There are no kinks in the lines, and during testing it squirts out pretty well. What do you think?