Frustrating I know but can be due to a number of reasons;

Possibly identified as out with your normal pattern of spending. Barclaycard were the first to use the software and it's used by all the major card companies to protect fraudulent activity, albeit December / January spending patterns can fluctuate due to Xmas spending.

Declined transaction response ;

Whenever a cardholder uses a credit or debit card to purchase a service or product, that Credit Card's Number is either entered into a website for an online Transaction or swiped at a Point-of-Sale terminal for an in-person Transaction. The credit card information is then sent to the bank of the merchant, or the Acquiring Bank, to the Issuing Bank, or the bank that initially gave the card to the cardholder. Using an Authorization Code, that Issuing Bank will check to see if the cardholder holds sufficient funds for the Transaction, as well as ensure that the card is not stolen or fraudulent. If the cardholder’s account does not hold sufficient funds (or if the card is stolen or fraudulent), the Issuing Bank will send a Declined Response back through the Acquiring Bank, and to the website or Point-of-Sale terminal; this message will alert the merchant to decline the transaction.

It would appear there is an automatic "declined response" being generated for your (attempted) purchases. Several attempts at the same merchant may also alert the software to potential fraudulent activity.

Lastly, your wife has maxed out the card on your Xmas presents. ��

Suggest you get back onto Amex.