filper, have you done a complete scan with your tuner?! Not just a "add digital channels" partial scan?!?
I've discovered that some tuners will skip scanning a channel, if the tuner had previously detected an analog signal (or another signal) on that channel. This allows the scan to proceed faster, but has the obvious consequence of missing changes (such as those that occur during a digital transition).
From what I understand, your installer does professional work, and it doesn't seem likely that your location or equipment could be the issue. To eliminate the setup, you could do some testing with a cheap set of rabbit ears and a balun. Amplification wouldn't be required, and you should get results even indoors.
DMX 68' tower, HyGain HAM 5 rotator, Antennas Direct 91-XG & C5, Channel Master 7777 preamp, Siemens surge protection