There can be allot of things wrong.

Read the tutorial on this site and then look at the code example in the code library. Compare your code with that of the tutorial.

1. Do you init the display correct?
2. Do you toggle enable at the right moment?
3. Do you use the RW/RS signals correct?
4. Does your P0 need external pullups or polerization?
5. Does the display you use support the Hitachi commando's?