One problem I've had is getting to a "sticky" point in the building sequence and putting off doing it. Then the momentum is lost and it's hard to come back and deal with whatever put me off in the first place. Sometimes I'll switch to an easier area of the project to get going again, then come back to the problem. Another is losing interest in the whole project. I've let one sit as long as 2 years until finally it looked interesting again and got finished. It sounds like a lot of us end up with unfinished projects cluttering up the place. Another good motivator is cutting off your kit buying funds until the current one(s) are finished! Sometimes it's so much easier to buy them than to finish them!