This is not really an issue with the Remote Control. As you said yourself, it worked before. Code for devices do not change, and this is a very common box with lots of other people happily using it and controlling it.
So it looks like you have a problem with your IR signal not reaching the IR receiver in your 3400. There could be lots of reasons for that, check out this thread: https://community.sling.com/message/10363#10363 for my ideas.