Online services tend to change their functionality or interaction. Sometimes in a good way, other times in a bad way. Most recently LinkedIn made a little change in the way accepting (and replying to) invites works. I personally use LinkedIn as a address book and leave it alone at times, never inviting people but combing trough the invites. I used to reply with a text-expander snippet too see if a connection is valuable, but this seemed to be impossible lately.