OK, we have to identify the actual problem, and the type of bike.

I gather it's a single speed, but please confirm. Also say whether it has rear suspension or not as this makes a tremendous difference. If it does have rear suspension, a photo will be very handy, or an accurate description of the pivot points and linkages.

Now the problem.

Is the wheel moving forward causing chain slack?
Is the chain tension OIK, ie. slight visible slack at ll times, yet the chain falls off?
What else should we know?

A chain can derail even if everythins is spot on if for example the sprocket has a damaged tooth, the chain line is off slightly, or the tooth profile isn't right to pick up the chain as it approaches. This will be worse on a jump bike because the chain is bouncing around and can deflect sideways, then snag and climb up onto the tips of the teeth and derail.