Ever since I've bought my Macbook Pro, I've noticed that it does not zoom/unzoom on the trackpad the way it does on other Macbook Pro's. I've only had the computer for about a year, and the zooming via trackpad has been enabled. I pinch/zoom once and then it doesn't let me do it anymore. So, essentially, if I pinch to zoom I cannot pinch to unzoom or even to zoom again. So it's stuck after one. Help please!