I am guessing maybe your left handed? Try tipping the phone the other direction. (left as you are looking at it.)

Also check this setting...

Menu - Settings - Sound and Display - Orientation (This should be checked)

 

I'm a righty and it sometimes can take a couple tries to get the phone to change to landscape, which hopefully the 2.0 update will fix. Anyway, I'll use a clock analogy to try to help you fix this: I've noticed that if you're holding the phone upright (top of the phone pointing towards 12 o'clock), if you skew the phone towards 2 o'clock and then flip it left back towards 9 o'clock, it tends to flip to landscape mode with no issue.