I will tell you right now, there is not a problem with that filler. Look else where for the cause. First thing, did you "kneed" the hardener? If you squirt some hardener out and some watery stuff comes out, that filler will NOT harden. I have had that happen to me a number of times in the past. I don't know what causes it, but it happens. Second, what brand is the filler? Some fillers are not very user friendly and the resins come to the top. When you sand it the darn paper will clog up like the filler is wet. Third, was the filler stired up good? If you scooped some out and got a bunch of resin with it, it ain't gonna harden well either.
You really need to find out what the problem is, because a bad filler would be REALLY, REALLY rare. In fact, I have NEVER seen it.
