If this should be in a better location, mods please move it.

I have been spending some time at the range, dividing time almost equally among my Glock 19, S&W M&P, and SIG 226. What I am finding is that with the Glock I shoot to the left and generally a bit low, 9 o'clock to about 7 o'clock. Not changing my grip (or anything else that I can think of) I am dead-center with the SIG and S&W, though my groups with the M&P are a little scattered.

Thoughts on why I might not be as good with the Glock? Maybe the sights and not me? Maybe entirely me and my grip? Ideas? Trying to troubleshoot a little.