I wouldnt run any brass that wasnt chamfered & debured through my dies, especially if you have just trimmed them as well.Are you chamfering the outside of your necks?
I'd guess your hitting the deprimer pin rod lenght adjuster.
Wind it up futher the rod & try that, even if you need to remove the depriming pin & pin tensioner if need be.
In saying that its weird how its just on one side.
I presume you have taken the die apart & cleaned it, looking for any inpefections around the neck forming/sizing area??