a) the carefully-designed addictiveness of Facebook
b) housework
c) child interruptions
d) fear (that the work will not be as good as it ought to be)
A, b, and c can all be addressed reasonably easily with tech solutions, scheduling, a partner who carries his share of house/child labor, and occasional application of money to babysitters.
But d is the root cause of all my writing delays, I think. Fear leads to avoidant behavior. Willpower can carry the battle on occasion, but willpower is hard. I think habit is actually the best tool in my arsenal. It's 10 a.m. on a Mon / Wed / Fri -- time to get a cup of tea, go to the laptop, turn off Facebook, turn on music, and open the file. This is the sequence, and the sequence shall not be deviated from.