How have you connected it, 8bit or 4 bit mode as the intialisation is different, also are you controlling the R/W pin or have you tied it low (you do not need to be able to read from the display to use it, you simply wait a short time), you may also be trying to send data to the display too fast for it to respond.

Search this forum for similar threads as it's been discussed many times before.

Richard

PS there are several web sites that cover the use of these displays search with google.