I don't know what model you have, but those blades are never really sharp even new. I have ground thousands of pounds without sharpening them. Replacement blades can be had for $6 or $8 and is probably the way to go...
The hobart should have a number on it Like the one I have which is a #32 and then you can go to cabelas, or homebutcher.com and get new plate and blades
You cant just sharpen the blade you have to grind the plate flat again,