Psychohistory presents a number of challenges that almost defy recognition. The behavior of human beings on and individual basis AND within the context of increasing complicated social organization. Here a few subjects that, I think, presents a bit of depth to the problem:
*Is society static (stays the same) or dynamic (constantly changing)? There are examples of both.
*Is human society based on biological characteristics or learned behavior? What is the interaction between the two?
*Is history a deterministic trend or is it entirely based upon preeminent leadership?
*Is there some "mystical" purpose to history or is just a series of accidents?

These are important issues that need to be addressed at some point. I do not see Psychohistory as some "science" to be discovered, but as an objective to be achieved.