If you notice that some of your products didn't get imported into your Yusp database, it is most likely due to some properties not being displayed on all of your product pages (i.e. some of the items in your stock do not have a product image).
You can resolve this issue in two general ways:
Go to the "Integration" tab on your Yusp Dashboard, scroll down to "Product structure definition", click on "Advanced", then set all the problematic properties to "Optional". This will enable Yusp to import items into your database that do not have the "Optional" properties, only the required ones.
On the "Integration" tab, you also have the possibility to identify product pages by defining a URL regexp or some jQuery selector that's present on all your product pages. You can do this under "Integration" > "Pages" > yourpage > "Advanced".