I have a Case IH FM-750 GPS unit. I use it to measure acres and locate points in a field from my 4 wheeler. For some reason I can not get it to measure acres now. It has a satellite signal. It will measure speed. When I hit the button to begin to measure acres nothing happens. I should have tried to locate a point but I have not. Is there a setting that needs changed? I am unaware that I accidentally changed anything.