How to manage dull employees?

That is a tricky question and it is tied to the organization as a whole. Not to answer a question with a question, but what type of organization is it?

Not for profit, government,etc.

Any recent layoffs, downsizing? If they feel they are doing mindless bot work, or more work for the same pay as last year, they are most likely just waiting for the end of the day.

Expanding their job authority and accountability is a start. What type of work are they doing?

I would recommend getting the book Gung Ho, by Ken Blanchard, (audio book if possible)

Try and introduce some competition and celebrate success. Put some time aside every week (say an hour on Friday afternoon) to get staff to go and do something sociable.