ive got a glass bow and dawnbreaker favourited

when my glass bow annoyingly gets disarmed against a draugr deathlord, switching to dawnbreaker causes it to do no damage and make a *thunk* sound - like im hitting a soggy piece of wood.

is this a known bug or am i doing something wrong here?

how do i stop it happening because ive died twice due to suddenly being unable to do any damage?

(im not using mods)

(second weapon functions perfectly against the deathlord until i get disarmed, and works again afterwards)