For quite a long time I have been bugged by this bug (;)). I have a solution with several projects in it and some of the projects have data sources. The problem is that sometimes when you switch to another project (e.g. by opening this project’s file in the editor) the data sources are empty. Now, there is a simple trick which can help resolving this problem (restarting VS doesn’t help). Open up the .xsd file from your project tree and rename anything there. Save it. Rename it back and save it again. Now your data sources should be once again visible in the data sources view.
Hope this helps someone.