What version of Blender are you using? Last I knew, the Better Collada plugin does not work with Godot 2.8.
However, Artell on Twitter posted that they ported the Better Collada plugin to Godot 2.8:
https://twitter.com/artellblender/status/1098521410685845504?lang=en
If you want to use the Better Collada plugin with Blender 2.8+, I would recommend checking out the linked GitHub repository and see if the new version works better. If that does not work, I would suggest installing Blender 2.7 and using that to export the Collada files. That is the setup I currently have, and it works decently most of the time, so long as I am not using Blender 2.8 specific features.