Syncing through client fails

Hi there! I keep having a problem with syncing files: some files sync fine, others don’t. I can see in the client that it has not synced (orange cloud) although it has been set to automatic syncing. When I click on ‘sync now’ behind the folder and use the existing folder for that the client replies ‘failed to add download task:
Repo already exists’. How do I put the automatic syncing back on?