Most common reason is that there is paper would around the cutters, imposing an additional load on the machine. You will need to access the cutter assembly (disconnect all power) and manually remove all of the paper that has impinged between the cutters.
If you have run any labels or paper with gummed strips through the cutters, you may wish to consider throwing this one away and buying new. (In all fairness, it "can" be cleaned, but it is going to be a real dickens of a job to do).
If you decide to keep your shredder and clean the cutter assembly, remember to oil it on a regular basis per the instruction manual.
As for me, I use Fellows shredder oil
Fellowes Shredder Oil 12 oz Bottle with Extension Nozzle 35250
and oil my machine every time I dump the cuttings. I run a bead of oil in a serpentine pattern over a sheet of paper and then just shred it. (If you have never oiled the cutters, it may be too late to do so now. Something you need to do on an on-going basis).