It is also easy to get tired of working on your thesis/dissertation problem.
You may also read a dead end or a point of frustration.
Some ideas (from [d, section 3.2]:
Set up regular meeting with your advisor
Through out an unrealistic schedule ("Finish 1 chapter per month")
Find the fastest method to test out an idea or theory, without investing months of time to find out it is wrong.
Focus on achieving a well defined, small goal (possibly leading to a chapter)