I have a bad schrader valve on my AC's low side service port. I went to one of the local auto parts shops and got a core removal tool for 134a cores. The core tool doesn't seem to be grabbing hold of the old core. Though when the core removal tool is pushed into the service port I can see, what seems to be, the core turning on the bottom of the service port but it never actually comes close to being loose. I'm wondering if anyone else has had this problem or if there's a special tool I need to get it loose.