It happened to me, first time I ever tried mechanicals. I just opened the package, screwed them onto my arrows and went to the broadhead target. None of the three heads opened. I checked them out and found out the blade slots were full of machining chips and the blades could not open. Couldn't even pull them open with pliers. Once I removed the blades and cleaned out the blade slots, the heads worked fine.

Sent a nastygram to the manufacturer and they sent me a fresh pack of heads, a cap and a couple dozen replacement blades.

The second pack of mechanicals I tried (different manufacturer) weren't quite as bad, but they had burrs around the screw holes in the blades and they didn't function properly either. I had to grind off the burrs with my dremel before the blades would flop open freely, like they're supposed to.

Both sets opened just fine and did what they were supposed to do, after I'd worked them over. Both brands were very popular heads at the time and were well spoken of on the forums. So, my experience was just my bad luck, or maybe The Man Upstairs was telling me I was not meant to be using mechanicals. Not good for someone who was skeptical of mechanicals in the first place, but was trying to keep enough of an open mind to try them out myself.

But the point is, always check mechanical heads out and make sure they work properly before hunting with them.