I recently had an issue whereas my predefined replies were not working when attempting to post a response to a user. In addition, any changes I made to my predefined replies were not being reflected in the drop-down menu when attempting to select a response. I upgraded to 3.20.x and had the same issue.
This indicated perhaps a cache issue, however my files and cache folders had proper permissions and cache was disabled in my general settings anyway.
The way I found to solve this was as follows:
1) I enabled my cache in General Settings.
2) I deleted all files (except index.html) from my "files" folder.
3) I checked my predefined response menu and it was empty - progress.
4) I added a new predefined response hoping to update the cache.
5) It worked.
Hope this helps somebody else