There is a minimum resolution requirement for each product when you are using Lab-Fulfillment. This is to prevent low resolution photos from being sent to the Lab and resulting in poor quality Prints.

If a product is not available for purchase, but it has been added to the appropriate Price Sheet, it means the photo you have uploaded has not met the minimum resolution requirement for that product.

For example, a 16x20 Print from WHCC and ProDPI requires the photo to be at least 1024x1280px and for Miller's and Mpix it must be at least 1600x2400px.

If you Preview the Collection, you'll see which products will not be shown to your client:

You can see example minimum resolution requirements here:
Lab Fulfillment: What is the minimum resolution required for products?

It might also be helpful to check that you have the correct Price Sheet attached to the Collection, in case you have accidentally linked a Price Sheet which contains no products:
How do I change the price sheet attached to a collection?