I had an issue when installing a DDX (Direct Drive Extruder) to my ender 3. It was not stable. The extruder kept wobbling, and I could quickly move it up and down, which was not normal. I tried tightening most of the bolts holding the mainframe of the extruder mount to the belt, but none of it worked. I googled it and found out that there is a bolt underneath the bottom wheel. It was slightly hidden. When tightened enough, there was no wobbling on the extruder. Be careful. Don’t tighten it too much. It can cause extra strain on the belts.
This helpful video helped me to find out the issue: