Check the Users Account and validate the Override Add To Cart setting is not enabled. This setting will hide the add to cart button for all users logged into this account.
Note: this may be controlled via an ERP integration if you have one so you may need to validate if the account should be purchased by when logged in.
Check the User Role assigned the the user does not have the Add To Cart Button unchecked.
Check the Supplier rule(s) to check if the Buy 0 Priced Items toggle is disabled along with the item missing the add to cart button has a $0.00 list price. This toggle when disabled removes the add to cart button for any products assigned to the supplier with a $0.00 list price.
Check the store Inventory Settings and Inventory Rule settings to ensure there is stock and the Hide "Add to Cart" button if no stock is not enabled. If this is enabled and there is no stock the Add to Cart button will be removed.
If there is no stock also check to ensure the Include products without inventory if the above is enabled as an item with no inventory records will inherit the Hide "Add to Cart" button if no stock setting is enabled.