Thanks for your replies. I did some Googling from your comments, and I see that there are indeed two ADSB-out frequencies in use, 978 and 1090. Well, that’s certainly interesting. And maybe the C150 was too low to be received by the open-source receivers in the area. I was not aware there were multiple frequencies.
The Webtrak site is interesting, seems to be able to track both frequencies, but that site is not available over the whole country.
Thanks!