Bitdefender can overreact to apps it does not recognise. If it decides that WinStitch is a problem on your machine, it can prevent us installing the threads and palettes and other support files. (We place those in Documents - it’s not supposed to be a ‘protected’ folder, but BitDefender seems to feel it should be.)
If this happens to you, here is how to enable WinStitch for BitDefender. 
(With thanks to Beth Davis-Hofbauer who worked with us patiently to find the cause)
Open BitDefender.
Click ‘View Features’   Under ‘Safe Files’, choose ‘Application Acess’
If you are having trouble, WinStitch will appear on that list , showing as ‘blocked’. Change that to ‘Allowed’, and all should work properly!