Compression ring not holding top of fork in place I just replaced a fork with a different one (Bluto»Mastodon) and, after inserting the compression ring into the top bearing (it has a tapered section), the steerer tube still wiggles from side to side. The bearings are new so there's no chance they are damaged. The entire set-up worked just a few moments ago without any problems, but now I cannot seem to figure out how to get it to stop moving. What could be the problem?

Here's a photo:
There's often a very thin intermediary washer/spacer piece that goes between the compression ring and the top plate. Some headsets have it and some don't, but on one that was intended to have it, leaving it out would cause the exact problem you're talking about, as the top plate would bottom out on the bearing cup or headtube before it makes contact with the compression ring.