When is a StubIndex built?

Hi,
when will a StubIndex be updated with the information of a certain PsiFIle?

I have an inspection which uses a StubIndex to check elements in a file. It accesses a StubIndex for the current PsiFile and for included files. The StubIndex contains entries for the current file but misses the entries of the other PsiFiles most of the time. How can I make sure that a StubIndex contains the elements for a given PsiFile / GlobalSearchScope ?

(I've asked two other questions about stubs, no reply so far.)

Thanks!
Wallaby