Square attracted plenty of attention for its mobile payment technology when it launched last month, but it looks like the company has been running into some difficulty getting its credit card reader out into the world.

In a letter to users sent out today, titled “The Home Stretch,” chief executive Jack Dorsey acknowledged that the company “released parts of Square before they were fully baked” but said that the entire team is working to fix the issues.

Specifically, he quoted a support email:

Until recently, we were facing a big hardware shortage, but that is now resolved (we sent our co-founder Jim to China for a couple weeks to arrange better manufacturing, and that did the trick). The problem has transitioned to something we’ve been working on simultaneously, a credit processing and risk issue. We need to strengthen our underwriting infrastructure so that we can handle the huge demand for readers and still manage the risk of chargebacks and fraud. This is the last thing preventing us from shipping readers as fast as we’d like, and we have pretty much the entire team working on it.

Square’s device is a small plastic credit card reader that plugs into an iPhone, an iPod Touch, an iPad, or an Android phone, and it also offers a way for users to enter their credit card numbers manually to make payments. Investors include Khosla Ventures, Google’s Marissa Mayer, Twitter cofounder Biz Stone, angel investor Ron Conway, Digg creator Kevin Rose, and others.

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