youtube autoplay in playlist does not work

When a song ends in my playlist the counter keeps counting time like 4.21/3.30, which is uncommon, and it keeps staying at the same song, it won't move on till i press on next song.

does anybody know how to solve this issue, i read couple of solutions, like send youtube to the background or check "plug-in savemode" from settings but i haven't found any helpful answer yet.

yosemite 10.10.3

safari 8.0.6 (10600.6.3)

newest v. of adobe flash