Knowledge Article is a new concept in CRM 7.0 version and I am glad to share my experience.
A Knowledge Article got created through data conversion and you have maintained all the mandatory fields including the Problem Description note types and other mandatory fields.
The problem is if you have created this Knowledge Article using a data conversion program, sometimes it will not be searchable in UI (except if you use the Posting Date as a Search Criteria) unless you open it in change mode in UI and then save it.
Only those Knowledge Articles which were created in UI frontend will be available for search using Knowledge Article ID and all other Search attributes. When you save a Knowledge Article in UI, the sytem makes additional method calls other than standard CRM_ORDER_SAVE to make it searchable in TREX.
Kindly refer to the below wiki for more details