1. The default /products and /collections URLs do not work
If the default /products and /collections URLs are not working, please go to Settings -> Permalinks and click "Save". This will manually flush your rewrite rules and should fix the issue.
2. Add to cart not working
WP Shopify will not work if you don't have the correct sales channel enabled for each product. This sales channel will be enabled by default after creating the private app during the syncing process. However if you're running into problems with the add to cart buttons not working, be sure to check that this is indeed enabled. The channel will be the name you gave your Private App during the connection process.
3. Unable to change product or collection slugs
Unfortunately Shopify does not allow you to change a product or collection slug after it has been created. Therefore you must delete and re-create your product within Shopify to change the slug. Once this is done you can resync from the Tools tab within the plugin settings.