When you load a multitrack project, Soundtrack Pro may not locate clips that have a slash ("/") character, also known as a solidus, as part of their name.
This can happen in situations in which a clip name includes a "/" character that is not preceded by a space. For example, a clip named "1/2 speed noise effect" would exhibit the issue, while one named "Half open / half closed door" would not.
Avoid slash characters when naming clips.