I have got one the same and in the field it has been a bit temperamental. Sometimes it beeps at thin air and other times it wont beep at all. When it plays up I hold down the plus sign until it ramps up and then take it back down (negative button)...this seems to stabilise it. I find with mine it does not have much range (distance) for small targets but goes wild on the bigger stuff.
Good luck
Good luck