I'm not an expert (there are lots of others on the Dunsire forum better qualified than I), but it sounds to me like it more likely might be a reed issue, especially if others are having the same problems with your chanter/reed. One of several things could be going on: it's not seated in the reed seat well (loose), the reed is chipped or split, it's too wet or it's too dry (some reeds like just a little bit of moisture).

It could also be that the reed is simply worn out (you made it too easy to play by shaving/sanding the blades). This weakens the reed and causes premature failure of the reed. To try to fix that, shave a sliver off the reed blades - no more than the width of the blade you use to cut with (about 1mm or less).

You can also check out Chris Apps' website to help in diagnosis of the problem.

There are lots of high-ranked competition bands using poly chanters that get great sound, so don't get discouraged.