How to instill fear of the known terror to players? (and other assorted gripes)

This may be a tad long, but here goes. As a long running DM, ive noticed that with the particular group I play with they have no sense of fear towards the known terror BBEG. To clarify this, its the fear towards a particular individual or power, in contrast to an unknown terror such as a being beyond the mortal realm or an untouchable aspect of nature. All that aside, if I were to place an enemy before them such as a powerful warrior or an old but powerful witch the group will not be fearful of their power but will openly mock, jab and banally argue with them. It feels like to me that they have no respect for the challenges I put before them and it makes me a bit disheartened as a DM.

Another gripe is that the group seems to have no respect for authority and like before, will openly mock, jab and poke holes in anything someone of superior status would say. Even with consequences met for their actions, they still continue to act the way they do and I don't really know what to do.

Any Advice?