What is your hboot version, radio version, and the hboot version of the PC36IMG.zip that you're trying to flash? Keep in mind that if you have an older hboot, but newer radios, you cannot flash a PC36IMG.zip with the older hboot, as it will attempt to flash older radios once you have S-ON. This is not possible. If that is the case, you have no choice but to either:
a) Get S-OFF again and manually downgrade radios, then go back to S-ON, and flash the older PC36IMG.zip, or:
b) Flash the PC36IMG.zip that matches your radio version rather than your hboot version. This will upgrade hboot, though, so if you care about that, go with option "a".
If this is not your problem, then it's a different issue. This could also be caused by using a bad sdcard, or specifically, one smaller than 8 GB.