When building a Flex library project in Flash Builder 4, setting the Include all Classes flag in the library properties tells the compiler to include all compiled classes into the resulting SWC file. Sometimes this flag gets reset or ignored. The .flexLibProperties file looks like this:
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<flexLibProperties includeAllClasses="true" version="1">
<includeClasses/>
<includeResources/>
<namespaceManifests/>
</flexLibProperties>
If I go in and set the flag by hand it works until the next time I checkout.