In addition to what others said... Dont expect it to land everywhere perfectly. Earth can be pretty rocky. I try to find a mostly flat area, get in close, and then use CALS. Trying to land on the side of a mountain is not good.
Remember that, for a while it was debated whether a CC should even be able to establish planetfall-- and there were some good reasons why they shouldn't. Ultimately, you can still take them planetside, but you have to remember that CCs are friggen big, so you have to take more care in how you use them. You cant trust the A/P to do everything right.
My suggestion is, find a nice flat area to land on first, and CALS should work every time.
In the end, things like this, coupled with your own imagination, help make the game more fun in my eyes.