you give little information

How about your settings in the administration - do you generate thumbnails at all?
What do you have in the template?
What version are you using, what template?
Where is the picture of the administration settings?
Aren't you using an extension?
Do you use lazzyload?
How about any JS error?
Do you check product detail.php and place where is call for additional images?
What about css file - exist code for style additional images?
Where is URL your shop?
Why are you hiding this information?
Did you know that without this information, your question is just a cry into the dark? == with zero value?
So far, I can just say 100% of your information that the sun will shine tomorrow == you just wrote it doesn't work - well ok, it doesn't work.
I'll wait for more information, you write as much information as possible in the meantime.
Check the settings of the folder where the thumbnails are to be saved, check on FTP whether the folder exists and whether thumbnails are generated there ...
Just do something yourself and not that you expect everything a stranger will solve everything for you, I do not see your efforts and it is frustrating ....
domain/administrator/index.php?option=com_virtuemart&view=config
TAB Template
Setup for Media Files Settings