1. How long is the cable from your anteena to your converter box? Approx. how old is the cable? Does it have any weather damage?
2. Check all connections starting from the antenna, to the amp, to the TV.
3. When was the last time you did a channel scan on the converter?
4. Disconnect the amp and connect the cable directly to your antenna. Try another channel scan.
5. See
Antennas Direct | Troubleshooting Problems with HDTV Antennas for some tips.
As you try new things, remember to do a channel scan each time on your converter box.