I'm in the middle of the same dispute with UA. Both TATL legs credited miles quickly, both CA/USA legs did not, appealed, got denial letters and following up. According to UA's and AC's website, there is a distinction between domestic (ie intra-Canada) and international. As CA-USA is international, just like TATL, and as it's all one booking and hence fare class, UA has no leg to stand on. Persuading them of this is harder.