Hi. There has been discussion of match problems back for some time on the discussion boards. Someone on one of these, provided support for ONE possible problem being certain ALAC (m4a, but Apple Lossless Audio Codec) files. (There are probably multiple problems, but I'm just guessing)
I too have had sketchy Match performance over the years but it's gotten better. I'm not complaining. But when iTunes recently as indicated in the article, and after an update, started to Match my library again, it hung up on the last files added to the library. Which happen to be high res ALAC files converted from FLACs. Having Match hang was new to me, *my problems had been isolated tracks not matching*.
The ALAC files it choked on were high res files at 96 kHz bit rate and 24 bit, converted from FLACs. Notably, ALACs I previously ripped from CDs at 48 kHz and 24 bit samples size have come to Match ok. (though there were problems early. They often didn't match at first, but they didn't hang the entire Match process, though.
Both the 48 and 96 KHz ALAC/m4a files sync and play just fine, but the 96 kHz files appear to be what hanging up MY iTunes Match. I could either/both downsample or remove the songs from my library, but I don't want to do that.