To answer your question, I would like to paraphrase some points made by my friend Whit Kelly on the subject. In a blog post he gives four common reasons for delays:
• Incomplete design databases
This is the almost common causes of delays. As I mentioned in some of my blog writings, you really need to have all the elements of the design finalized before going to production. Leaving even the smallest element open to interpretation will lead to the supplier rendering it in the way exactly opposite from what you wanted.
• Payment delay
China is pretty close to a cash economy unless you are a buyer for a Fortune 500 company. All purchase orders will require a deposit, in most cases because the manufacturer is going to use that money to pay cash for the raw materials. Tooling is typically 30 percent at time of order, 40 percent at first article and 30 percent at final approval, before delivery. Production is most often 50 percent at time of order and 50 percent at time of shipment, which means in practice, 100 percent is due before the product leaves the factory. All these terms are clearly communicated early in the project, but at least 80 percent of the time, there are delays from clients not paying in a timely fashion.
• The uncommunicative buyer
It is not uncommon for projects to get caught in the doldrums and go nowhere for days, weeks, even months because the client is AWOL. It is very, very common to play "hurry up and wait."
• The buyer who is in a hurry
While this might seem to be the right attitude to keep things moving fast, the buyer who is in a hurry often places speed over accuracy. They want to cut corners, skip prototypes, skip writing a Product Quality Manual, not bothering to proofread the PQM, etc. Sometimes this works out, but not often. More commonly when you deviate from the normal project flow, you end up causing more delays.
China vendors mess up all the time, but the delay is not all the vendors' fault as the four common pitfalls above explain.
Luckily, it is easy for buyers to avoid these pitfalls if they want to keep sourcing projects moving forward on schedule.
For reference, you can read more about Kelly's Laws of China Sourcing.