Sometimes, the SKUs do not appear in the list when you search them on the Restock Inventory page. Also, there is a problem syncing such SKUs. This is a common issue due to the invalid SKU format.
Shopify APIs do not work well with special characters or spaces in the SKU name. As per Shopify guidelines for SKU formats 'Don't use special characters, symbols, or spaces, because these can cause problems when searching for products in Shopify by SKU and in other computer programs, such as spreadsheet software.' You can find the guidelines here.
Only dashes or underscores are ok. Removing spaces and other special characters from the SKU names should resolve the problem.