I recently experienced something similar. It happened simply because the dropdown menu had expanded and therefore the size of the menu was larger. Try to resize the menu back down to a single line by clicking on the bottom and dragging it smaller. This then resizes it back to the original and should then display the “Show more” and “Show less” buttons.
Just to explain; the logic is that “show more” expands to the maximum, and “show less” returns to the last position that you dragged the panel to manually. So, if you drag the panel to it’s maximum, both the “show more” and “show less” have no real function any more.
(There were some discussions, at the time this was implemented, about other options, such as “show less” should take the user back to the minimum, or to one row less than max. I think the conclusion was that if the user drags to the max they are aware that they can drag the panel back, so there was not such a strong need to build extra complexity in the logic in order to try to guess what users might expect).