If you are using PiAware, and your MLAT results suddenly are not appearing on your dump1090 local web interface. Check your MLAT configuration.
SSH into your PiAware device and issue the following commands:
sudo piaware-config -mlatResultsFormat "beast,connect,localhost:30104 beast,connect,feed.adsbexchange.com:30005" sudo piaware-config -restart
This tells PiAware to send MLAT results to ADS-B exchange, as well as the default local port.