the problems that i have seen run the gamut...but generally you can break down a lot of problems into vague catagories..
low end marks (under 200 us) mostly sears, and valves.
Mid to high range- most problems dealing with markers that run in the 400 plus are due to operator error, lack of maintenance, etc....very rarely will it be something with the marker itself it seems...unless you just happen to get a problem marker....some markers just will not work no matter what oyu do to them...and some will keep working in spite of what you do to them too.