Currently this is where I have my mobs dropping for my xp generator. Every time I leave the area and return a few of the mobs will have escaped. This happens for every type of mob I have put in so far, currently it has wither skeletons and as much as I love fighting them, it really sucks when you are at level 118 and get killed. So far I have tried expanding the area they land in in every direction (originally they landed in a 3x1 area, instead of the current 5x1) and that didn't prevent the escape. I tried blocking the sides with whole blocks and that didn't prevent it. It seems so far everything I have tried still allows the mobs to phase out when I leave the area and come back. I'm hoping someone has a solution so I can stop getting killed.