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.
The top part of the truck is screwed to the deck and is called the baseplate, and beneath it is the hanger. The axle runs through the hanger. Between the baseplate and the hanger are bushings, also rubbers or grommets, that provide the cushion mechanism for turning the skateboard. The bushings cushion the truck when it turns. The stiffer the bushings, the more resistant the skateboard is to turning. The softer the bushings, the easier it is to turn. Bushings come in varying shapes and urethane formulas as well as durometers, which may affect turning, rebound and durability. A bolt called a kingpin holds these parts together and fits inside the bushings. Thus by tightening or loosening the kingpin nut, the trucks can be adjusted loosely for better turning and tighter for more stability (useful when landing tricks). Standard kingpin nut size is 3/8" - 24tpi. The position of the hanger respect to the baseplate is also determined by the pivot, a rod that slots into the corresponding seat in the baseplate. The pivot stops the hanger from rotating around the kingpin. The pivot must allow some movement around the bushings and therefore is not a perfect fit. The space between the pivot and its seat in the baseplate is filled by a pivot cup, a plastic part that will take most of the wear and tear of the pivot and assist in centering the hanger needs to be lubricated every so often.
A rushed take-off might be the problem here. Steps 2-4 of this trick tip could help you out. In order to make your feet “stick” to the board, you have to do the movement fluidly. But don’t stress yourself out! You should rather start with some preliminary exercises until you feel safe while ollieing. It is also important to pull your foot up to your board’s nose in a straight manner to prevent it from spinning.