I believe I've solved my problem.

This has to do with the org units. Our customers are imported into Service Desk database with one set of "org unit" fields. Other items have been imported with slightly different names for the same org units. A human can understand the intent and interpret both sets of org units as containing the same information. However, the Database sees these as different org units so customers from "Org Unit 1" are not able to see Items from "Org Unit1". Having realized this, I should be able to work on cleaning this up by normalizing my org units across the board and/or setting the org units on items that should be seen by everyone to "All" (ie remove all org units from the item so that All org units and Everybody can see these items).