I had the same problem with loose T7 FFg the other day. Hangfires with No.11 caps on a sidelock. It's a can I bought two years ago, but the lid has been kept tight and it was stored in a dry place. I had to install a Magspark nipple and change to No. 209 shotgun primers. This stopped the hangfires, and accuracy was good. But I use Pyrodex in the same gun with CCI No. 11 magnum caps, and ignition is good!

I don't recall this problem with T7 from the same container when I first bought it.