Mini Watchface previews (pictures) are not shown properly in 'My Watches'

Although I installed the Watchmaker app on my new smartphone and copied all my existing watchfaces into the proper app folders, only some of the so called 'Mini Watchface' previews (pictures) appear properly in the 'My Watches' section with the dedicated mini preview picture. - Some 'Mini Watchface' previews only show the default app symbol (a white watch symbol on a red background) :-/ - If I tab on such an app symbol, the correct watchface appears ( in big) and I can work with it. - But as soon as I close the watchface and return to the 'My Watches' section, the default app Symbol (white watch on red background) re-appears instead of the 'mini preview' of the new created watchface. - Any ideas how to solve this problem ?