snow leopard 10.6.8, itunes 11.4 will not add files to the library

If the problem is still there, try booting into the Safe Mode using your normal account. Disconnect all peripherals except those needed for the test. Shut down the computer and then power it back up after waiting 10 seconds. Immediately after hearing the startup chime, hold down the shift key and continue to hold it until the gray Apple icon and a progress bar appear. The boot up is significantly slower than normal. This will reset some caches, forces a directory check, and disables all startup and login items, among other things. When you reboot normally, the initial reboot may be slower than normal. If the system operates normally, there may be 3rd party applications which are causing a problem. Try deleting/disabling the third party applications after a restart by using the application un-installer. For each disable/delete, you will need to restart if you don't do them all at once.

Safe Mode

Have you looked in the Recently Added folder? Often when people complain about this it is because the files are not tagged and don't end up where people expect them to be.

Okay, Apple does another questionable "recommends" (nothing against you Eric Root.). 😟

Safe Mode won't hurt anything but I see this issue every other week or two here on the forum for the past decade in helping here and I can't ever recall when starting the computer in Safe Mode was the solution. Often it is the case the tracks do not end up where the user expect them to be because of lack of tags, or a simple computer restart will help, or this particular set of files are not compatible with iTunes, or handful of other reasons before you get to it being a corrupt cache or third party utility.