Try taking the water line off at the top and reattaching it, giving it a small twist if you can to make sure it seats well. A very small leak anywhere above the water line could cause your problems.
I read that someone also gave the filter sponge a good squeeze to remove bubbles. Air was trapped in the filter and was slowly releasing into the water.
I personally noticed bubbles for about the first month or so and they seemed to diminish slowly over time. I still get a few and have no clue how they are forming. I have to look very close to even spot them, so I just don't worry about it.