Assuming that your MMM worked fine before installing the new blades, what if anything have you disassembled/reassembled beyond the bolts and blades?
Can you now freely move the blades by hand, ensuring free travel?
Assuming the above has been checked, verify that your drive belts are correctly attached to your drive spindles, in the correct pattern, and correct tension is felt.
The Make/Model of tractor and belly mower would help as well.