Have you ever had your credit card declined? It has happened to most people at one point or another for a variety of reasons, including President Obama when he tried to use it at a New York City restaurant this past year.
Credit card companies are increasingly looking for potential fraudulent transactions and will decline any charges they think are suspicious. This often occurs when people are traveling. For example, if you use your card in your home state in the morning and then use it that same day in another state, it can look suspicious. This is particularly true if you are traveling to a foreign country where credit card fraud can be more prevalent. The best way to prevent this from happening to you is to notify your credit card company about your travel schedule before you take your trip so they will not decline these charges.
Another situation that is fairly common is when you funds are on hold by a hotel or other company that has not yet charged your account. These holds can count against your limit and prevent you from being able to use all of your credit. Once again, the best thing that you can do is stay in contact with your credit card company so you know how much available credit you have before trying to make a major purchase, and can prevent your charges from getting declined due to hitting the credit limit.