if a monster is sitting on the edge of the screen (edge fo a nametable actually) there are issues with the objects and their attributes.
in my demo, damaging a monster sitting on the edge of a nametable (maybe 8pixels wide area or so horizontally) are effectively invulnerable as they are in a different table than the player is and simply refuse to register damage.
funny enough they can damage the player just fine. sounds somewhat the opposite of the issue you're having but im quite sure it's related. For me it randomly happens in the middle of the screen since the scrolling has the nametables move around
but in a single screen enviroment, this happens exclusively on the screen edges. (i simply go around this for now by using a destroyMe edge reaction, until i manage to get something done for better handling of off-screen monsters.)