I know that when we got a harness, both of our cats had the exact same reaction: They would not move. EVEN if we got their treats and shook them. We didn't have to be holding on to the harness/leash. It just had to be on them for them to start freaking out! If we DID try to walk them, they would both flop like they were paralyzed.

We tried using the harness, trying to get them used to it, about 20 times each. After that, we figured there was no hope and tossed the thing.