Lately I have come up consistently with low OG readings. I do partial boil so it seemed logical that it was a poor mixture problem with the wort and the added water and I was right. I thought that I was mixing the wort/water up enough but yesterday, as I was getting ready to take a gravity sample, I noticed that the wort at the bottom was slightly warmer than that at the top. After doing a lot more agitating, I took a gravity sample and it was consistent with the expected value. I now have a new agitation method that seems to work better.