Capital & Cash flow. Whether it is start up capital or bridge loans to get over a cash flow problem due to slow payers, lack of enough capital will cripple or sink a business.

You have to over estimate your start up capital. There will always be unexpected expenses in any start up. This is why 90% or new businesses fail in the first 5 years.

Even if you make it past that, there is the need for periodic extra capital needs. It could be bulking up inventory to meet a new great opportunity, but then they are slow to pay you. Or extra payroll dollars to meet a new project. You have to pay employees immediately and may not get paid 30-60 days out. Can you cover that when it happens so you do not have to turn down big fluctuations in your business. 

One of the ways I have worked through this in the 20+ years in business, is to have flexible payment plans available. Especially for new large projects I will negotiate with the client an early payment plan for a discounted price. For example, 2% discount if invoice paid withing 2 days of receipt. This has worked great when I explain that I am not a bank, and if I have to borrow money to meet their needs it costs money. So I am willing to share the savings with them if they have the ability to pay early.