I have logged into a website based on Websphere Portal 6.0, gone to the Administration console, then to Portlet Management -> Portlets.
At a guess, there are about 200 portlets on this server. 22 portlets have a title starting with "IGM", but if I use the "Title starts with" search option and enter "igm" (or any case variation), only one of these portlets is found.
The portlets excluded from the "Title starts with" search are also excluded from the "Title contains" search, but some of them can be found with the "Unique name contains" search (noting that only 5 of these have a unique name).

Why would title searches exclude these portlets?

I also see similar behaviour in other areas of the Portal administration.
E.g. going to Portal Settings -> Custom Unique Names -> Pages, and performing title searches excludes some results, depending on the search terms entered.