hi andre,
there are different things that could went wrong e.g. the new piaware uses another port for mlat results (30104) than older versions of piaware and dump1090 use (30004) - see this thread http://discussions.flightaware.com/ads-b-flight-tracking-f21/piaware-2-1-3-released-t36047.html. you could look into your piaware log
cat /tmp/piaware.out
and post the result here.
what is your actual dump1090 version? there are older versions that won’t display mlat results.
another option - if you have a second sd-card - try the ‘plug and play solution’ sd-card image
have fun
tom
p.s. regarding your antenna you find very good tips from user abcd to improve reception without money: http://discussions.flightaware.com/ads-b-flight-tracking-f21/three-easy-diy-antennas-for-beginners-t20177.html