An easy way to check for a venting issue would be to just take the cap off the tank and see if it keeps running. If that helps at least you know where to start looking. If it doesn't I would check the supply issues stated above, elect pump, colapassed hose, crimped line or filters. I just started mine after being down for a year and had a lot of water in the tank from condensation as it had only 1/3 tank. I would also drain the bottom of the filter bowls and tank to check for moisture.
Good Luck,
littlebob