Digital tuners vary in performance.
You believe that the aerial signal is OK, but it might just be sufficient for some tuners to work and just below the threshold for the others.
In an ideal world, you would have a signal that was a minimum 20dB above the threshold, which would allow for variations of the signal caused by weather conditions etc and differences in performance of digital tuners.
You could try it out by purchasing a 20dB attenuator and fitting it inline with the aerial feed and seeing if you still received an error free picture and sound.
Noise pick up can also cause errors and a digital approved aerial with a balun and good quality coaxial cable will alleviate this.