I have migrated my website form Magento 1.9, i have manage to import all products and orders but i have a problem. When i add a new product, it won't show the thumbnail on product page, there is no image created on that link, if u click on the default magento thumbnail it will show the big gallery image ok.
The only solutions that i have is to copy all the images from the gallery to the thumbnail folder.
I have tried the usual deploy, cache flush, cache clean, reindex etc and even finished image resize command but w/o any luck.
For the migrated product i had the same problem but fixed it by copying the gallery folder to the thumbnail folder.
I'm using Magento 2.3.4 version and i migrated from 1.9.