This can occur on occasion. Typically, it means that the top & straw (and bottle) are in need of a good soapy cleaning. Follow standard cleaning procedures which are on the instructions that come with each bottle. In some cases, it may be necessary to soak the top/straw for a bit before cleaning, or use a fine brush in the straw. Typically soaking in soapy water or vinegar water takes care of any build-up inside the top/straw. Rinse thoroughly. Again, it is important to periodically clean the bottle, top & straw to help keep this from occurring in the future.