There is a 50-point skill cap for each profile. If the limit has been reached, you may need to remove some to add more. In addition, if you remove skills from your profile page, the endorsements associated with those skills disappear. Therefore, you must re-add your skills. When you re-add them, their endorsements will also return. If you are certain that the issue is unrelated to the first two solutions, your connection who endorsed you may have removed it. However, there is no way to discover it due to LinkedIn’s privacy policies.