When I set the drone on the ground, my app always says its lower than it is. Normally -8.5. Could this be a reason that drone doesn't take off 9/10 times?

why don't you turn the drone on AFTER you put it on the ground? also let it warm up for a minute if it doesn't start.

Hello Ktretter26, the height value has nothing to do with take-off. Barometer performance will be influenced by air pressure and air flow.But if your drone can hold it altitude and attitude well in the air, you don't need to worry about the negative value too much.