For each profile, there is a skill limit, which is 50. If you have reached the limit, you may need to remove some of them to add new ones. Another reason is that if you removed some skills from your profile page, your endorsements under that skill disappear. Thus, you need to re-add your skills. When you re-add them, your endorsements will come back too. If you're sure the problem is not about with the first two solutions, maybe your connection who endorsed you removed it. However, there is no way to learn it because of the privacy policy of LinkedIn.