Seasonic S12 500W

The Seasonic S12 series is widely regarded as the quietest consumer-grade PSU on the market. I presume thats why you bought one?

I would generally assume that this means the fan is a low RPM model and as such probably has a relatively low CFM rating.

If you can see the fan rotating fine, I can't see how it would be an issue. Try disconnecting the case fan, and put some thin (eg tissue) paper over the PSU fan vent. Does the paper move? This should give you a good indication of whether its moving air or not.

Many PSU's (particularly the quietest ones) only ramp up the fan speed to a significant level when they reach a particular temperature threshold (which may never happen in your case). I'm not sure if this applies to the S12 but it wouldn't surprise me.

If the blades are spinning and there is literally *no* expelled airflow then this would be a symptom of having a fan the 'wrong way round' (eg sucking). However I've never heard of a PSU being shipped like this lol, certainly not a respected brand.

Here's a little something to do for emotional reassurance:

Take a piece of tissue paper and cover the honeycomb on the back and then try the same thing with the fan grill. You will see suction on the fan grill and blowing out the back (although these will both be small when the PSU is lightly loaded.)