If your Shopify orders are not being imported to Printful, please check if you have set up Shopify to capture payments automatically. Printful only imports orders with captured payments. Here's the Shopify manual on how to do it. We’ll still import your order if you choose to capture the payment manually.
If the issue persists, check if you’ve selected Do not automatically fulfill any of the order's line item in Shopify admin > Settings > Checkout > Order. If so, your order won’t appear in your Printful Dashboard until you manually request fulfillment and Printful accepts it.