Apparently, about half of the people who start dissertation programs don't finish. It's not new research, but I'm trying to compile my own list of problems--my own perspective--to guide my efforts.
Also, I just like classifying stuff.
Generally I would break the world of problems up as follows:
1. psychological -- mental habits or emotional issues that interfere with writing
3. personal/non-academic--stuff like having to support yourself, care for family, etc.
4. interpersonal/academic --problems with the faculty committee
5. theoretical -- a poorly thought-out study can be very difficult to work on
6. practical/academic -- a well-thought out study can present practical limitations that prevent completion--being over-ambitious can be a problem here.
Off the top of my head that about covers it. I think it probable that most dissertation writers may have more than one of these problems.
Being able to understand the problems can be useful in making a plan to get done with the dissertation.
What problems are you having?