There’s an automatic “backoff” feature of Windows Indexing from Vista onward, which causes it to slow down and stop depending on other load. This can prevent Indexing from working altogether when load is high and/or searchable datasets are very big. To fix this, change this registry entry:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows Search\Gathering Manager\DisableBackoff
to a numeral 1. You will probably have to change the owner of “Gathering Manager” to Administrators in order to do so. After this change is made, restart Windows Indexing.