Hamlet

Some productions cut it as extraneous. Those that use it will often occupy Claudius with some feigned banter as if he is not paying attention. But as to the question. We don't know why Claudius doesn't react. The play does have two purposes though. One is to catch Gertrude and one to catch Claudius. The Dumbshow goes by relatively quickly and in some sense is abstract in it presentation that there is not enough impact to deliver a real knockout. Because it goes by so fast and is not reinforced we have to know that Claudius is made of sterner stuff and is not going to unkennel his guilt on such a passing slight. I think it does prime the pump though and sets off his reaction at the end of the play within the play.