Have you filled the back portion with water? A lot of HOB's are no stronger than they have to be, especially "kit" filters. If the back is not full and it has to pull a full air vacuum, it may not be strong enough.
The instructions usually tell you to "prime" them by filling up the back with a cup or some such, but if you're like many of us, you've either forgotten where you put them or tossed them some time ago.
