I've always found it interesting how on many aircraft it takes a few days in the sim to handle it in abnormal conditions, but ages of real experience to milk the best out of the aircon.
Now, casting my mind back a few years, I seem to recall the following regarding the ATR:
Don't have the temp selectors at minimum. They are effectively a cabin temperature demand selector, not a supplied air temperature demand selector. In theory at least. In practice if you set them close to minimum you do get colder air, but potentially not as much of it. Try tweaking the temp selector UPWARDS and watching the supplied air temperature (I forget what thats called on the ATR. is it 'Duct temp' ?) You should sense a substantially increased flow rate before there is any significant increase in the suply air temperature.
i.e. to push it to extremes, your options are a lot of cool air or a little bit of very cold air. The former is the best bet for overall cooling.
The other thing that can happen in the ATR is that ice can form in the duct and choke off the air flow. In this case,again selecting a higher temperature is the way forwards. The ice will melt and come flying out like a little snow storm (providing a cooling sprinkling of ice particle onto the lucky recipient!). Once the duct has been melted out, start nudging the slector down again.
In summary, somewhat counter intuitively, ATR high cabin temperatures can often be attacked by turning the temperature knobs UP.
(Caveat: Haven't flown one for 7 years or so. Intervening years effects of FTLs have addled my memory completely so it everything I've written above may in fact be complete cobblers.)
pb