I think I know what happened to Yammy.
"After all those humans and Shinigami kill all the other Espada, I Yammy will be the number one Espada! And I will rule all other hollows!" Yammy thought.
So now he's hiding in Nobodycaresville for his chance to strike and take the "Primera" spot.
But, srsly what happened to this guy? I know if he fought he'd literally get killed in like 5 seconds, if he fought Ichigo* anyway, so did he just go die somewhere or what?
*B'coz Ichigo cut off his arm in literally like 2 seconds, so to kill him I'd doubt it'd take longer than 5.