We need more details about your system.
How many pumps, a description of the inside of the filter box.
Was the filter box assembled correctly?
Does the filter box have it's own pump or does it share a pump with the waterfall?
If it has it's own pump, something might be jammed in the impeller or the impeller shaft could be broken or installed incorrectly.
Too many things can be causing the problem. Without more information, we're just guessing.
Pictures would also help.