Where Quadremesh gives up

[encephalon]

occasionally it happens that quadremesh would not work, here the threshold seems to be 224 faces, lowering the faces by 1 and further it starts acting up, at around 200 it does not connect the object anymore at all.

adaptive quad count check does not change the result just in case you want to try

Quadremesh fail.3dm (2.0 MB)

[nathanletwory (Nathan 'jesterKing' Letwory)]

I see this in 8.15

Anyway, getting disconnected parts probably means you should up the adaptive size. That way the algorithm can ensure pieces still get connected (at the expense of probably higher final quad count than requested at target).