Had a similar problem with mine in both 223 and 30-30. Turns out it is very primer sensitive, and soft primers tend to flow back, making it almost impossible to open. I ended up pushing the open lever against the wood bench rest, hard, until it opened. When you get it open, check the primers. If they appear raised, or have "swipe" marks on them, try a different primer, such as CCI, which tend to be on the hard side. My 30-30 barrel did this with factory Winchester ammo, but not Remington, or hand loads with CCI primers.
Larry