Weird Playing problem
 

Sometimes when I put music into the playlist it won't play the last song I entered unless I highlight it in my library. But that brings up other problems cause then it won't play the songs in my playlist right. It just plays the highlighted one in the library. Sometimes it just takes a little playing around with but it's annoying. Is there anyway to fix this? I think just unhighliting would work but I can't get it to do that. I just got a mac so I'm not sure how things work. Thanks