[amsat-bb] AO-51 SIGNAL?

Hi All, I've been noticing that when AO-51 passes me, I have a much harder time receiving a decent signal from it. It does not matter what type of antenna I have ( beam or vertical). But it's much tougher copying a voice signal after the bird passes . I'm doing the doppler shift, but it's still quite hard. Am I doing something wrong? 73 de Doug ka8qcu