There is one of two things going on.
1 is that you need to update from the same site you activated it on.
2 is that a few versions ago there was a problem with the automatic update not working. https://support.advancedcustomfields.com/forums/topic/acf-pro-update-bug/. The fix may or may not work for you. I needed to update most of my sites manually to get it working again. Go to your account and download the newest version and upload manually https://www.advancedcustomfields.com/my-account/
