Hi Arash,
I'm really sorry to hear that you did not get the ATAS clearance that you applied for. Looking on the website it seems that you have to actually contact them in order to get the instructions for making an appeal against a refusal or if you want to find out the reason why the application was rejected. They do say that one reason for rejection could be that the info in the form did not exactly match the info provided by the university, or did not match the info on your visa etc. They don't say what the other reasons could be. It is probably worth getting in touch with them and asking them about this.
I wish you all the best.
Milly.