In the beginning, there were a few issues. Sometimes depending on when you ordered, two-day shipping might not actually mean two-day shipping. Order on a Monday then you were most likely good, but order on a Thursday, and you’ll have to wait the whole weekend to get your order.
The other major problem with Prime was that the costs of shipping for Amazon were rising to the point that Prime was losing the company money. Amazon began tweaking the program in 2006 to make it work better for companies and consumers alike. The addition of add-on purchases and Prime Standard Shipping helped to curb the loss of money. Though the cost of shipping had greatly increased for the company, the rising popularity of Amazon Prime made it possible to expand the number of fulfillment centers Amazon had and is still building across the US.