Why can't my customers reach the checkout page?

I have a couple of customers (very well known people to me) who have not been able to get to checkout. In the latest instance, the person tried to add 3 to her cart, but at first it would only allow for 2. I increased the number available to 10 (it had previously been 5), but could not find any other setting issue. After that, she was able to add the 3, but it would not get her to checkout. Where else can I check for the problem?