I had this problem as well (Purchased through Steam). I had the beta/alpha versions installed before the final release, but my follower stopped following when the final build came out on the release date.
I tried everything I could think of to fix the problem, including re-installing the game completely, but it didn't work. I noticed after I uninstalled the game that there were still game files in the '\Steam\steamapps\common' folder.
I deleted the game folder under the Steamapps folder, then went in to '\My Documents\Larian Studios' and deleted the 'Divinity - Original Sin' folder as well, and finally re-installed the game.
After doing all of this, my characters were following me as they should. I hope this helps anyone else with this problem.
