One thing it could also be is the falling block safety has some dirt or grease that is not allowing it to fall out of the way quick enough. This is usually more troublesome in cold weather. I pour boiling water over mine to clean out all the oil and grime, and then relubricate. Only had it happen to me once, but that fixed it.

As explained above, given that you are fireforming, headspace is the most likely culprit.