[Koha] Item not available at the OPAC display for some records

Hello All, I am running Koha 3.04 on Ubuntu 10.10 I noticed that when i search on OPAC some of our records are showing Items not available mostly Journals and Reference items, and for those that are even available the correct number of items are not specified and some it shows checked out but when clicked on the Title, the item is available. How can i set the system preference to correct this. thanks