I had similar problems recently:
1) Replace batteries in Meter if your readings keep fluxuating or are going off
2) Most likely your meter is the problem as was mine-kept giving me false readings even though it PHed correctly in solutions-go figure-think about buying new meter!
3) Best way to check Ph of soil -water soil when it is dry with PHed water of 6.5-6.6, wait half an hour for water to bind with soil then water lightly with neutral Phed water (7.0)=just enough water to come out of bottom to take reading
Again, my money is on your meter being off
Hope this helps-Smoothy