Skateboard slips from underneath me when doing smoother push offs

This is actually the exact same effect that makes a car burn out. Just instead of being stuck with spinning wheels, you're board is taking off leaving you to do the splits.

What's happening is actually the result of a combination of forces and differences in static and dynamic coefficients of friction between the board and your foot and between the wheels and the pavement.

To solve this you have some options:

• stick to using shorter pushes and don't change anything with your wheels or grip tape or stance.

• choose a stance that increases the contact of your foot with the board in the direction it's slipping. make sure your foot is long ways on the board so that you have the maximum amount of friction against the direction of motion.

• Make sure you are bearing down on the board hard. The reason that the chassis of a car doesn't take off during a burnout is that it's attached to the heavy body of the car. Increase the pressure you are putting on the board with your board foot during a push off.

• Change the combination of grip tape and shoe grip you are using. Not sure how to advise here, but it's a potential issue and one to look at if the first two solutions

• The last thing to check is whether or not you need different wheels for the surface your riding, a skate shop pro can help you make a good decision on that.

This is a skill that you can definitely learn, and getting your technique right is the first step. Turn that foot so the to is pointed in the direction of the board and bear down. If that doesn't work start exploring whether you can find a hardware solution that helps you solve it.