I sharpen mine with the blue circle sharpener I got from JoAnn's. I've never had a problem with sharpening.
I also buy my blades from Harbor Freight, they last a long time and I sharpen them before I use them because they are really for rug cutting. Sometimes the blades from Harbor Freight have nicks, but I just sharpen and they are fine.
Cost are 2 for $2, can't beat that price.
Hope this helps