Assuming you used expander ball in resizing, there are 2 possible causes I can think of.
1. The bullets were too much lopsided when the seating started.
2. There is burr at the casing mouth.
Similar thing happened to me a few times. Very rarely nevertheless.
I'm also big fans of .303 British, and I own one SMLE #1 MK III and one SMLE #4. All in original military configurations, and shoot well. I'd love to have a #5 if the missus allows.