Just an update, Nitrogensports.com and their support are great. They were in fact making prompt payments (verified by the blockchain), but I was having a "shared wallet" issue at CampBX, which nitrogensports warns about on their website.
Basically, the bitcoin deposit addresses at CampBX are not for your personal wallet. What happens is that when you send bitcoins to those addresses, they are received by CampBX, and then they are supposed to credit your account.