We'd like to help. When you try to add videos to your watch later playlist, do you see an error message? For videos that are in your subs feed, do you see this happening across devices? Try signing out and signing in, let us know if this helps.
Have you tried clearing your browser’s cache & cookies to check if that resolves the issue: https://goo.gl/HZi6cx ? Also, try using Incognito mode or a different browser the see if that makes a difference. Let us know if this works.