Ron:I also have the slide in tool. I noticed the other day when pulling a handle on my '57, if I left the slide in tool in place, the handle wouldn't come off. If I used the tool to push the clip back, then removed the tool, the handle came right off. So I stuck the tool back in the slot and looked, and the two "ears?" of the tool that pushed the clip out protruded into the groove and wouldn't let the handle come off. I don't know if your '55 handles are exactly like the '57s. I know that my tool won't work on my '53 truck, seems like the slot in the tool isn't wide enough. Been a while, maybe I'm not remembering right. Maybe comparing your tool to the handle you already have off will tell you what's happening.
John