did you by chance get a package of the (recycled) blades?
some shops recycle blades- they are deemed still usable for cutting carpet- which is what they sell them for- the carpet cutters allow alot more pressure to be put into the cut-so the blades are not necessarily that sharp. i think they do say on the package (recycled)
if that's the case you might be able to return them for new blades. depends on the store