Accessibility and scheduling of trades.

I once spent a day with the head of a large homebuilding company--hundreds of units at a time. Much of his day was spent on the phone checking with sub-trades and suppliers--alternately cajoling, blustering and urging them to get workers and supplies on-site. The balance of the day, he drove from site to site in his pick-up for an in-person view. Sometimes even hauling needed supplies around himself.

And here I thought it was only just lil ole me and my one lousy house or two that was having a hard time!

You've asked this question before and I'll give you my answer again: the length of time for a task doesn't matter unless its on the critical path. Home builders are often not trained in the use of such management tools and can't always identify the most serious scheduling issues until its too late. Subs who don't show up or who finish late can start a chain of delays that is difficult to avoid. Larger projects tend to use larger subs who are better able to meet planned and unplanned scheduling situations.

Other problems are incomplete design documents and inadequate supervision.

Homebuilding is a tough job. I would be surprised if you found any simple solutions but I would study what is done on larger jobs and see if it can be modified to work for smaller jobs. I would like to see an AIA Cost of the Work Plus a Fee Agreement with integral General Conditions that is half the length of A104 specifically for single-family homes.