If you drag and drop that parent folder onto the library and then make
yourself a sandwich, does the player find out about these files?
The player won't automatically readd deleted files, since that would drive
you crazy.
If the library is corrupt--
http://zachd.com/pss/pss.html#medialibrary
would fix you up. The easiest symptom of that is that the Monitored Folders
list is empty.

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Speaking for myself only.
See http://zachd.com/pss/pss.html for some helpful WMP info.
This posting is provided "AS IS" with no warranties, and confers no rights.
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> Windows Media Player had been able to add files to the library instantly,
> but
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> manage each one of them (there are about 300 subfolders).
> So how can I make it add files in the subfolders again ?