So, I found a really great slime chunk in my world a few months back and dug it out and made it into a slime farm. Now, this slime chunk was spitting out slimes at a pretty decent rate. It was great. However, today I went to harvest some more slimes and didn't get a single slime in over half an hour. Nothing has changed since the last time I was down there. It even was fine after the last update.
Are you sure there was no update between your last visit and now? The same thing happened to a number of players with the last update, including myself on a 360 world, and they've said it will be fixed with the next update.