Chapter 4 - spoilers - help me pls

Hello ! i have started the fourth chapter of the first game. I am fighting against the mysterious man and i have been dying multiple times during the fight. I have noticed the blood pockets aren’t working. Could you please tell me how to use them? I have tried to put them in Sam’s pocket but it doesn’t do anything and I have even looked it up on Google but I don’t get why the blood pockets aren’t working during the fight.
Any help is welcomed please !
Thanks in advance