Without a little more information .......

Broken decapping pin . Nothing to push the primer out .

Berdan primers . No hole for the decapping pin to push the primer out and you now have a broken decapping pin or it's pushed up too far to hit the primer and push it out .

Adjustment of the die and decapping pin . Lee dies sometimes require , to me anyway ,an uncomfortable level of torque to lock the decapping pin in place and it moves surprisingly easy if it isn't locked down . If not a Lee die you may need to adjust the decapping pin deeper as it may not be reaching far enough to get all of the primers free of the pocket . This can be made worse by small flash holes and sealed or staked/crimped primers .

Occasionally you will get a piece of brass where the primer cup stretches , the head is thick , the primer pocket is very tight and there just isn't enough reach to get the primer out . In probably 50,000 pieces of 40 or so cartridges I've had maybe 10 pieces of WWII 06' do this .

Last and probably the most unusual but more common than the last one , is to have a primer stick to the decapping pin hard enough to be pulled back into the pocket and partially or even completely reseated .