I'm assuming you're doing serial programming. The datasheet states that the maximum rate for SCLK is 1/16th the XTAL speed. So if you're running 24Mhz, the SCLK speed should be no more than 1.5Mhz. If you're running a slower xtal, SCLK needs to be comparably slower.


Beyond that, there's a bunch of timing rules to follow. Apply power, pull reset high and SCLK low, wait 10ms before doing anything, then issue the command to enter programming. Are you attempting byte mode or page mode? Are you waiting 4ms per command?


Check out page 38 of the datasheet.


Rob