It seems there's an issue with friend requests between Xbox One and Xbox 360. If you "friend" 360 player using the One, you'll be only a follower instead of getting the true friend connection. 360 users will only see a pending friend request that never gets completed.
See this for a further description of the issue:
http://hardforum.com/showthread.php?p=1041308238360 users can see their followers only when they log in to xbox.com. So I guess it is advisable for One users to execute friend requests via xbox.com, if there's no other way to get it done properly on the One itself.
That stinks, since there are some pending requests on my friends list for over a year now, and all the time I had no idea what was up.