IV Question

Hey guys, I'm a new AEMT and the ER that I trained at did me the disservice of using self occluding IV catheters. They said all you gotta do is hold pressure over the catheter and will stop the blood from flowing back. Fast-forward to me getting out in the field and it seems that no matter how many times I hold pressure over the catheter and no matter how hard I do it the patient's arm gets covered in blood. Am I doing something wrong?