Well, the lawn could be full of little metal. Mine was. Try taking it somewhere else and try it before doing anything else. Sometimes the coil wires go bad also.
I find my Ace 250 goes off real bad between my house and the neighbor's. I have 2 massive radio antenna maybe half a mile away and also a rats nest of powerlines. Maybe some radio frequency interference (RFI) ? I agree with smokey, find a nice secluded spot to test out the rig? Good luck and happy hunting.
PS I found an old swim beach that was undetectable due to RFI. Real bummer as it was a popular spot in the 20s and 30s..
Lowering the sensitivity and find a 'good" piece of ground to ground balance. Also, try shifting the working frequency from F1 - F4 and determine if the machine behaves well.
Buried pipes and power lines really F with my detector, sometimes you just gotta turn the sensitivity down. Also 22 blanks are sometimes used in concrete nail guns or less commonly starter pistols for races.