As someone who started a manufacturing firm around 18 months ago i can say without doubt the biggest issue is cash flow (but then any biz graduates should no that anyway). It is surprising how quicky a cash flow situation can change from one minute when the end is near to the next when your safe for another few months.
Hard to pinpoint one as I am still starting up myself, probably based around funding and guidance.
I think many startups probably have good ideas, the lack of guidance towards executing the ideas correctly is what causes the struggle.
You learn from your mistakes, hopefully we can learn from more experienced pro's instead.
I am trying to create a massive brand, but it is very easy to get distracted by long term vision and forgetting the important stuff today.
Biggest struggle : definitely lack of guidance in our economy for start ups.