Mobs ghosting through solid blocks

Chickens (adult) moving through solid blocks (glass, sand) in farm. Eggs dispensed into half block high space. When they mature, they suffocate in the solid block above. But now some are not suffocating and are laying eggs and then ghosting through the side walls which were initially glass blocks. I tried replacing those blocks with sand. Some apparently suffocate in the sand and emerge as drops. The majority of drops are collected but adult chickens are escaping and paying eggs while they are supposed to be suffocating.