Bad Capper?

I have an Agata bench capper, and on my last batch I had about 50% failure on sealing and maintaining carbonation. I keg my beer and carbonate in the keg, the bottle as needed for special events. This is the fifth case I have bottled and the first with issues. I moved to the bench capper because I had problems earlier with a wing capper and breaking/chipping the bottle tops.

Can I have some bad caps? Should the caps be warm and soft? Am I missing something obvious? I visually inspect all bottles before and after capping and they all looked identical.

Is there really no input on what might be going wrong for me? I'm grasping at straws now trying to figure this out.

i could be the capper or caps... i have a 1900era bench capper that is worn out at the spot for SN/Red Stripe stubby bottles and also the cup is loose so it doesn't fully crimp. i had to finish with my Emily hand capper; it has never let me down yet... try the oxygen caps at williams brewing and see what happens. they are all i use and not too expensive.