Many non calibrated NDBs at GA airfields use poor quality antennas which do not produce a vertical cone of silence, the radiated signal may be weak and bearings may also be affected.
Airfield NDBs aren't regulated calibrated like other Nav Aids, they're installed, licensed and thereafter, their output is monitered by the CAA.
If they start transmitting beyond their licensed power output, the operator will receive a call from the Department of Wigglyamps instructing them to tone it down.
Which leads on to:
Many ADFs at airfields are low-powered locators and are only effective when within 10nm or thereabouts radius and seem more susceptable to interference from Wx and terrain.
That hits the nail on the head, the licensed range of most airfield NDBs is only 15 - 20 nm largely because of the lack of available frequencies and interference factor from other NDBs (or even radio stations) in Europe.
The next time we get high pressure and suitable flying conditions, tune into an airfield NDB 30nm or further away, fly at 3000ft or above and try and track it accurately to 15nm.