I've had the same problem with Duxford and with Old Warden. On both occaisons I didn't see them until I was in their overhead (but above 2000 ft).
If I hadn't done it myself I would find it difficult to believe that anyone could fail to see an airfield whose location they knew and which they were looking out for.
I think a contributory factor was that as I approached I had reduced the scale of the display on the GPS. I suspect that during the flight I had got used to the larger scale and so was looking too far ahead for the airfield.
Now I try to pick a landmark near to the airfield that is easy to spot from the air and then visually locate the field in relation to the landmark.
Popham is easy to find because it is on the north edge of the biggest wood for miles around in the fork of the M3 and A303. Similarly with Compton Abbas. It's far easier to pick up the wood on the south side of the field and then the light beacon than it is to look for the field itself.
Mike