If you are still having problems, here's the next step. Go out and try to ollie, pulling those feet UP, and don't even worry about what your front foot does. You likely know that you should be sliding it up the board, right? Well, for now, just don't think about it. Focus on getting those feet up high, and let the board figure itself out. Try this a few times, and see what happens. Don't read more of this article - go outside, do this a few times, and then come back. You might fall. That's OK.
Some skaters get so focused on sliding that front foot, that they are actually pushing the board down with it. If you found your ollie was higher after that, then you are one of these people. That's OK - now you see what's going on, and you can focus more on letting everything happen with your ollies, instead of forcing it to happen. Go out and practice some more.
You are still going to have to figure out how to slide that front foot, and not doing so can also keep your ollies low. And, without sliding that front foot, you aren't really ollying! Here's the foot sliding section of the How to Ollie instructions, to find out more.
Still having problems? The truth is, ollies are tough to figure out without having someone there watching you, giving you advice. But even then, if the person giving you advice isn't good at explaining things in ways that make sense to you, you might still be frustrated.
Read "How to Ollie" for some detailed explanations, and take a look at the "Troubleshooting" section at the end. Ask someone you know and trust to watch you, and point out what's going on. the person doesn't even have to know how to ollie - have them take a look at the Learn How To Ollie article, and they should have an idea. Or videotape yourself, and watch it. Try different things - move your feet around on the board, try ollying while rolling, or while standing still. If you still have problems, then practice more, and drop by the Skateboarding Forum and ask for more help, or read other skater's stories on how they learned to ollie on the How Did You Learn to Ollie? Readers Respond page!