Trademarks must be continuously used to be enforceable. If you stop using them, they are lost. This also means you should have evidence to prove use of your trademark. If your use is challenged, you will have to prove that you’ve been using your trademark, and you need the correct evidence. This just happened to McDonalds in Europe. Even though they were using the BIG MAC trademark, they didn’t have the correct evidence to prove that they had continuously used that trademark for the previous five years, and they lost their European registration. A truly shocking result.