Peter Boulton wrote
typically I get 25% to 50% through the trunk and then the saw gets stuck
and I can get no further.
that's probably the weight of the tree closing the saw kerf. Get a
rope on the top bit and pull it so that the kerf is opened by the
bending, then try again.

For the ones without room to saw with a bow saw, try an electric
aligator saw, they are safer than chainsaws.