I’ve managed to figure it out just now (and there was no info online and cjdropshipping support weren’t able to help).
What you need to do, is go onto your shopify account and then go to “products”, click on the product that isn’t allowing your shipping address, then scroll down to “Available inventory at:” and select cjdropshipping.
Then click on each variant of the item (each size, colour etc) below that, and change the “inventory will be stocked at” location to cjdropshipping. Click save at the bottom and do the same again for each variant until they’re all changed to inventory stocked at cjdropshipping.
That has worked for me- let me know how you get on!