Why don't my IMs show up on my friend's computer?

My friend and I like to chat quite a bit. Now, one day, she sent me a message and I replied. Problem is, she didn't get it. She told me on the phone that it would say I was typing, and then it would be blank. I can see all of her messages, but apparently not vice versa. If it makes a difference, she uses MSN (Windows Live) and I use Yahoo! Messenger. Is there something wrong with either of our computers? We've been using the same Messengers from the beginning, but this is the first time that this is happening. Thanks in advance. :)