Just to make sure, did you link your fetchapp to your shopify store. Check at this URL below. Change YOURSTORE to your URL for fetchapp.
YOURSTORE.fetchapp.com/admin/settings/payments#shopify
Secondly, make sure that your SKU are unique for each product you want to import into Fetchapp.
I have found that sometimes if I have made changes to a products such as the SKU, Fetchapp won't update the products.
Are you clicking the import products button in Fetchapp?
Hope this helps.