If the circulation rules for an item type are set to 'no holds allowed', but a librarian overrides this rule and forces an item level hold, the hold request will never be filled. When the item is checked in, Koha searches for any open hold requests that this item may fulfill, but when it sees that items with this item type don't allow holds, it skips over it. This behavior should probably be overridden for item-level holds. The relevant code is in C4::Reserves::CheckReserves.
*** This bug has been marked as a duplicate of bug 11512 ***