I have heard repeatedly that many/most buyers are disappointed in cash flow in the first year or two after acquisition.

What are the most common reasons for this? Specifically, what are buyers missing in these cases?

Some that I can imagine:
1. Attrition of key employees leads to lower business performance
2. Customer acquisition (or revenue generation) was more dependent on the owner than the buyer realized
3. Buyer realizes that the business was understaffed at the time of sale
4. Buyer missed some material information during DD that triggers a decline (eg: a new competitor, a key contract is not renewed .etc..)
5. Buyer did not consider the full impact of taxes or interest rate changes in projected cashflow

What's real?