Breakin would be my guess. I've personally fired about a case of 00 8 pellet low-recoil FFC through my 1301 shotguns with no issues...but I didn't shoot flight control through them until after I'd fired a couple of hundred rounds of birdshot through them. Given my penchant for practicing with heavy birdshot loads, the springs saw some significant breakin prior to patterning them with FFC.
Definitely need to look into the ammunition (including how it was stored...shotgun shells can be more sensitive to temperature/humidity than metallic cartridges) and if that checks out contact Beretta and have them look at the guns.
It would also be worthwhile to go burn a case of birdshot through each of them and then try shooting the low recoil buckshot again.