Low ollies: This can happen for a lot of reasons, but the biggest one is that you are not crouching low enough before your ollie, and not pulling your feet high enough after you jump. When you crouch down, try and touch the ground. When you jump, try to hit yourself in the chest with your knees. ​Both knees. Don’t worry about falling. That will happen sometimes – that’s just part of skateboarding! For more help, read the How Can I Make My Ollies Higher? FAQ
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.