Has it done that with any ammo besides WWB? The very first time I took my M&P 40 to the range I had several times when it wouldn't return to battery. I was shooting 165 grain Fiocchi FMJ. Once I finished that box of ammo I switched to 165 grain Blazer and it functioned just fine. Since that time I've put hundreds of rounds through it with no problems. WWB, Blazer, PMC, Federal FMJ, American Eagle, various different kinds of high dollar JHP ammo all feed flawlessly through it. I haven't tried any more Fiocchi so I don't know if the problem was the Fiocchi or the fact that the gun was brand new and may have needed a few rounds to get it broken in. I suspect it was the ammo though since, as I say, the gun's been completely reliable otherwise.