Do the poster frames not show up at all, or are they static images (and you want the actual video playing in them).
If the latter, switch to the "Motion" tab in the "Properties" section and make sure the "Animate Buttons" box is checked.
It's been a while since I've used video buttons, but I never would drag-and-drop the timeline onto it; I didn't even know that was possible! I would select the button and use the pickwhip to link to the timeline and chapter, or I'd create the links in the Flowchart panel. Try one of those options to see if you get different results.
