As far as I can see, you check for a previous version in Line 176. Are you sure that this code is executed at all and what is the value of %PREVIOUSVERSION% in your test? And are you sure the uninstall is executed in your Line 194? Does your command line /U "[PREV_UNINSTALL_LOG]" /S passes the correct and expected values to the uninstaller? If the uninstall does nothing, then I think it does not even execute and that's why it does nothing.

BTW, I would not do all these uninstall actions from within the LOOP WIZARD. If I were you I would do it after the Progress Dialog: Display () command.

And your use of "Wow6432Node" in your registry manipulation is incorrect. Not sure if this already makes a difference if you fix this and then run the deinstallation procedure.