Wow. Thanks for all the responses. I did figure out what was wrong. It took me another 50-odd tries, but in the end it turned out to be a question of timing. On the ground and running, you hit the jump key while using the forward key. On the platform, I was hitting the forward key and then hitting the jump key. You have to hit them both at virtually the same time. I'm really glad i didn't have to cheat.
