Two points: Sometimes the object which has the focus is not very obvious in Poser, whether it be an actor in the scene Preview, or the library palette, or the animation palette. They all respond differently to a delete keypress and being sure exactly what has the focus is important. The other caveat/question is that the first keyframe at frame 1 can never be deleted. Poser just won't do it. But I'm confident that's not what you were referring to.

I've also noticed that the animation palette will not always retain the previous selection rectangle for keyframes after an action. Apart from prompting you to delete an actor or figure, it can occasionally be mysterious why the keyframes previously selected weren't deleted, until you notice that they got deselected somehow.

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