The timing when using these modules is critical. You MUST read the datasheet carefully! If you don't understand what you're reading, then that will most likely cause you a problem. First - they have a startup delay after power-on. Then you have the interface timing - using the newer faster cpus trip a lot of people up here. There are setup and hold times specified, when you output you data you have to wait a setup time before you set the DEN signal, on an old 8051 at 12MHz, this was virtually guaranteed by the speed of the cpu, go above 12mhz and/or use a 6/2/1 clk cpu, you're going to have to add delays. Same with the DEN high time and post DEN data hold time.

You also have the instruction delays - if the datasheet says it takes 160uS for the instruction - be sure you issue the instruction and wait at least 160uS!

Then you have the hardware issues - if you are using P0 on the 8051, it will most likely need pullup resistors.

Everyone who has used the lcd devices has most likely fallen for at least one of the above traps. Beware the sample code that says it's guaranteed to work - on what cpu?, at what speed?
Most of the sample code I've seen breaks at least one rule.

Welcome to the fun of embedded systems where the software meets the hardware!