Interception not working?

I was playing as Assyria and was at war with Austria. They attacked my CS ally with GW bombers and i had 3 triplanes in the city about 4 or 5 tiles from CS, but they didn't intercept. Yes, i had set them to intercept before that and yes, target was in range. Does interceptors ignore CS allies or is this a bug? I remember that interceptors did defend CS allies in G&K, or am i wrong?

I lately parked an Arabian carrier next to Vienna and my triplanes did intercept Roman bombers that were attacking Vienna.
Regarding your case, I think 4 or 5 tiles is to far away, isn't it? I don't know exactly how big a triplane's interception radius is. At least I think it is smaller than the attack radius.
Edit:
Triplanes have an interception probability of 50%. Maybe just bad luck in your case?