Youtube won't change video quality

i've been using chrome for a few years now and i never had this problem.

the problem is:

whenever i go to youtube on chrome and i chance the video quality it stops. the video stops playing. it doens't buffer. there is only a black screen. it doesn't matter if i change it to 720p or to 240p. it just stops.

now when i try to do the same thing on safari it works normally.

when someone uploads a youtube video on facebook and i try to change the qualitity when i'm on chrome it also works normally.

i only have this problem when i'm and chrome and on youtube.com

i updated my flash player. i made sure it was enabled. i reinstalled chrome. but after 2/3 weeks it still isn't fixed.

what else can i try? who has the same problem? how to fix this? what to do?

thanks