I may be missing something very obvious, but sometimes it happens that a lemma which is already in a project’s key terms list does not show up in the Biblical Terms Renderings panel (nor the Biblical Terms tool window). I will then attempt to add the Hebrew/Greek lemma by right-clicking on it in the Source Language Text panel (Add to project Biblical Terms for…) only to be denied with the error, “Cannot add Lemma, it is already in the list.” The reference in question will appear in the Basic → References box in the Edit Biblical Term box. What’s more, the Renderings panel will be set to display the current project’s biblical terms list.
For example, I encounter this problem with the Hebrew lemma פָּרָשׁ “war-horse, horsemen” (Ex 14:9 et al).