I am new to this forum and have no experience here, but a few questions immediatly came to mind. Is the firing pin getting full travel? Possible crud in the receiver preventing full travel. Did you do a thorough cleaning of the receiver before lighting her up the first time? Is it possible that the primers are sitting too deep in the primer carrier? Did you try a different primer carrier? How is the head space, at the breech? I don't know the spec for this on a ML or even if there is a spec for it on a ML. Did you try it with just a primer and unloaded...did the primer fire?
Yeah, I know, just more questions...just thought maybe you hadn't thought of checking or trying these things. Good Luck.