In Amavisd config file /etc/amavisd/amavisd.conf, change $log_level, then restart amavis service.
/etc/amavisd/amavisd.conf
$log_level
$log_level = 5; # verbosity 0..5, -d
If you want to debug SpamAssassin, please update $sa_debug also:
$sa_debug
$sa_debug = 1;