When it comes to keeping hate as a tank, you have some things to consider. If there are say 3 enemies on the tank and the tank is using his normal single attack skills mixed with flash, it will be difficult to keep hate if the DPS attack one of the two monsters the tank is not hitting with his attack skills even if he is using flash. So if a tank marks monsters it's important for DPS to follow the marks in order.

Another thing to consider is that ilvl of tank compared to DPS makes a big difference. If there's a ilvl60 tank with 2 ilvl 100 DPS, the tank is going to have a hard time keeping hate. It's possible, but considerably more difficult.

Lastly, the DOTs (damage over time) skills do pull hate. So if there are multiple monsters on a tank, it can be difficult for them to keep hate on all of them at the same time even with using flash. So in close, it may not be solely the tanks fault for not keeping hate sometimes. Go easy on us.