Slide your front foot as soon as you start jumping. Use the same movement you have been practicing to slide your foot towards the top end of the board, just as you jump off it and kick the back down. The front of your board should lift up as you slide your foot along it, with your foot hitting the top of the board at the highest point of your jump.
To summarize, a skateboarder's feet need to do two things successfully to complete an ollie. They need to provide a changing force to move the board correctly (so that the combined force of gravity and the skater's feet add up to the green arrows above), and they need to provide different amounts of force with each foot (shown by the red and blue arrows above) to steer and turn the board into the right orientation.
To get higher ollies, I would suggesting resting your back foot on the very end of the tail and the front foot in the middle of the deck to allow for more slide. Popping as hard as you can and catching your board in the groove of the nose with you ankle practically resting in the groove will level the board out in the air. after this initial movement slide you feet back to the bolts on the tail and nose to ensure a clean landing.