If you have any other available case/chassis_fan headers on the motherboard, you can try a different connector. Or, you can use an adapter and give the fan power directly from the PSU. Some fans need more juice than the header can provide so you may need to change how it's hooked up. Also, Antec makes good stuff but some of their fans are quite cheap. If you can't get it to work after changing changing the power source you might want to find a replacement.
There is also a chance that the BIOS is limiting the fan's speed but if that were the case I would think it would at least start spinning on its own... might be worth to check though.
Ah my mistake, but I believe Antec makes the ASUS fans anyway, but I could be wrong. If the fan used to work and now it doesn't, it may just need to be cleaned out. If it's been a year or so, depending on how clean the air is, there could be some build up of dust or pet hair. There's really only the one moving part on the fans so if there's something making it struggle removing it will fix it.
If you find that this is the case, I recommend after cleaning it to use a fan filter. This will help keep the blades and hub cleaner.
The motherboard BIOS may be changing the speed automatically depending on the temperature inside the case. There are sensors that monitor this and in turn adjust the speed accordingly. This is why you may want to set the speeds manually in BIOS or download a fan controller to use on your desktop after startup.
When I have any doubts at all about a fan I'll replace it, it's cheap insurance. None of them last more than 2 or 3 years (except Deltas and a couple other industrial grade fans), and the usual "name brand" crappy fan is lucky to last 6 months. I would feel stupid if I let a $10 fan wipe out my $1500 system. I am a bit of a fanatic about temps and airflow. If you have multiple case & CPU fans, one can fail without frying the system, you'll soon notice the dead fan (because of the elevated temps) and fix it. There's a total of 9 120mm fans in my system, if you count the 2 on the H100 radiator, but with good fan control, it's cool and quiet until you really stress it, and even then it's not bad.