TV Fool - Missing stations we could get weeks ago in Fort Collins, CO

Here's my TV Fool report:
http://www.tvfool.com/?option=com_wr...1dda162957c5a8

When we first hooked up our Clearstream 2 Complete from Antennas Direct, we could get about 39 channels, but we couldn't get NBC (9.1) from Denver. However, about 6 months later, 9.1 suddenly started coming in perfectly. It didn't coincide with anything like leaves falling off a tree in the way (there isn't one), so that was a mystery to us.

About 2 weeks ago we started getting interference in 9.1 and 7.1 (ABC) from Denver. It got worse, and as of yesterday we have neither of those channels. Again, nothing's changed on our end. We still get 4.1 (CBS) perfectly and it's nearly identical in power, path and dB to 9.1 and 7.1.

We'll go try adjusting the antenna in the next few days, but can someone explain why this might have happened for the curious?