I have mapped my farm into default and internet sites. Problem is, on the internet zone, some site collections are still pointing to the default zone which cant be resolved from outside. This causes some site collections to inaccessible from outside the intranet LAN. Why is this the case? Can I manually force the site collection to pick the external mappings?
AAM is not the same as URL rewriting. I usually try to appoint 1 URL for a web application as default and use that for both internal and external traffic. I use extending only for cases in which you need a different authentication provider for instance.
So try to use the URL which is resolved from outside as default and point internal users to the same address. Especially for collaboration purposes, people sending eachother links to documents, this is far more logical then using multiple URLs to get to the same stuff.