They exhibit much the same behaviour in my DW, and that has LWAMI on it. I had a Soveremmnyy (or however you spell it) hammer it past a line of four I'd
supposedly blocked an inlet with, the ship doing about 31 knots. It went right in between two of them making more noise than a squadron of B-52s on a carpet bombing mission and they just sat there doing nothing. It was within about 30 feet of one of them, so it must have passed through the seeker cone, as it was pointing directly into the cone. the only thing I could possibly think of, is that some mines are intelligent in the way they trigger, i.e. they will let a certain number of ships past before they detonate, as this makes sweeping and detecting them trickier. There are all kinds of variations on this theme where mines are concerned. however, I doubt that is the issue here, I think they are just defective, unless someone knows otherwise.
They're about as much use as a chocolate fireguard as far as I can see.
Chock