hi, i had the same problem (3.18.9 version); in the About Koha > System information i had warnings about the fact that i didn't choose between grs1 and dom indexing; switching from grs1 to dom indexing and doing a full reindex solved the problem. hope this helps.

I have recently entered more than three hundred item records in to Koha 3.18.07.000. they are not searchable on both the opac and the Staff interface, but can be circulated(checked out/ Checkin) using their barcode.