Went to a school yard I've been to many times and normally the ground cancels at around 65 to 75. Today, fast grab would not go below 90. Tried many different spots, all 90. I decided to try the manual ground cancel in all metal and the reading with just a slight positive was down around 35. So I left it on 35 and went back to discriminate mode did another fast grab and it would not move off the 35 setting I did manually. I then shut the machine off, turned it back on, did a fast grab, back to 90. What gives? Decided to call it quits, got home and did the fast grab in my yard and got the fluctuating readings I would expect in fast grab from 70 to 73. Is there just too much of "something" in the ground at the school to properly balance? 