Did ya trim to length ? I have had the same problem and found a few cases would be a bit longer than the rest , if i used the longest case to set the die it worked ok but grouped bad if i used a short one i would lose the longer cases as you decribed . If I trimmed all the same and camfered inside and out the problem went away and groups were good ! I do it on new brass now and don't lose any cases , before i sized and camfered and would lose a few cases !
good luck !