Import feature is not working properly and does not importing all items

[shopfy-2020]

For over two years, the import feature is not working properly. It's not importing all products even when all CSV details are accurate. 

I am exporting all products from one store to another, and sure enough, the same issues I've seen in the past continues. Even when the CSV file is exported from an existing store (meaning it's accurately configured) and imported to a new store, so many of the products are missing. When I check by brand, a whole bunch of products are missing.

There are no error messages. Support doesn't get it and constantly keep asking about emails with "error message". It's an internal server error, not an upload error.