We noticed something similar at Caribou, where there was freezing rain at one of our airports but we didn't see it in IRIS. It seems that these observations are only pulled in if they occurred in the latest hourly ob (I1 in the last hour, I3 in the last hour, etc).
I would also be interested in a way to tally these in a "storm-total" fashion so that the LSR doesn't have to be timed to the observations!