I agree with jinx, while magnums can be a PITA at times, they are generally pretty reliable.
Problems i have had in the that kept them from working...not being put together properly, whether its the shaft or impellar or are you sure you have the base on the right way? I struggled with that a few times when i first started working with these. If the hole on the canister doesnt line up with the impellar it wont work. Another thing that has made them do what you describe is being clogged. What kind of shaped is the filter floss pad and/or micron pad and carbon container in??? If its so clogged that it cant draw water...then well, it cant draw water and wont work.
Just a few suggestions, hth. Sorry about your troubles...i definitely know how frustrating that crap can be!