I have a xp1. When I first set it up I had to unscrew the top of the intake, and fill it with water until the intake tube was filled. Then I had to let it sit for two minutes and then plug it in. I have done maintainence on it every week since, and it did give me a bit of a problem once--identical to the one you are having. I just pulled the lever up (to stop the water), shook the canister and then pushed the lever back down (to start the water again). Sure enough a few bubbles came out and it was fine.
HTH!