With Physbones still being fairly "new" to VRChat, some newly uploaded avatars may have Dynamic Bones.
It is not a major deal if you import the avatar package without knowing if it has Dynamic or Physbones.
Simply close the avatar scene, import Dynamic Bones if needed and reopen the scene.
If you converted your Dynamic Bones to Physbones using the VRCSDK, it's likely some of your bones are not moving afterwards.
Find conversion tools online, ask for help in the Community section, or learn how to properly use Physbones on avatars.
(you do NOT need Dynamic bones if the avatar has Physbones already UNLESS stated by the uploader/comments)