Search Does Not Behave For Some Terms

Large Merged Project. I do not understand how search works in a merged project. Sometimes one term does not pull up topics from all the projects, which confuses my users. For example, only one project responds correctly when a user types forms, for example. If I type "forms" (with the quote marks), it behaves more like I would expect, but this is something extra to teach my users that is inconsistent with other terms. I cannot find any reason for this behavior. All the projects appear to have same settings. Often these missing topics already have forms as part of their heading 1. I have tried to force the missing topics to show by adding forms to keywords. This is just one example. Why does this happen and how can I control it?