I have a commode that most of the time empties as it should when
flushed. On occassion, the water does not want to empty. When this
happens, I usually wait a few hours and it flushes normally.

The home was built mid 50's and has cast iron drainage and venting.
Is it possible that the vent is clogged maybe by a bird trying to
build a nest?

I can't figure out why the toilet flushes most of the time, but on
occassion it refuses too.