There are certain industries that make it very hard to get reference customers. Say, you're in pest control and you want to show that you're the leader in killing everything that shouldn't be in fast food restaurants. Needless to say, you're going to have a hard time getting the big burger chains to admit that they're your customer.
There may also be certain industries that are very weary to sign a reference agreement. I've made this experience with government accounts. They just often feel they've got nothing to gain from being a reference customer and it's just too much red tape to make it worth their while.
There are ways to get around this problem. If you're the market leader, find an official top-ten list of burger chains and see how many of them are your customers. If there are several top-ten list, chose the one that works best for you. Without naming the chains, you can now safely make statements such as "we kill bugs in 8 out of the 10 largest fast food companies" and demonstrate leadership in that category.
If you are not a market leader, this may not work for you, but there are other options. Find out how many units you ship to a certain industry, for example you may sell 8 billion paper clips to federal government each year. Even if you cannot name the accounts, it's a great statement.
Whatever metric you use, be sure to document how you arrived at this figure in case one of your competitor challenges you in court.