Can you share with us the dough recipe you have been using? That might provide a few clues.
Itís also important to understand that there are differences between the King Arthur bread flour and the Harvest King flour. One of our members, November, performed various tests on the two flours and reported his results at http://www.pizzamaking.com/forum/index.php/topic,4008.msg37559.html#msg37559
(Reply 21). It may well be that you will have to make a few changes to the dough formulation if you want to substitute the Harvest King flour for the King Arthur bread flour. Having compared the specs of the two flours when the Harvest King flour hit the supermarket shelves, I can tell you that the Harvest King is lower in protein and appears to have a lower absorption rate. You will also note from this link, http://www.gmflour.com/gmflour/pflour.asp,
even General Mills does not include the Harvest King flour in its lists of Pizza Flours. It is included under http://www.gmflour.com/gmflour/flour.asp?type=EharvestKing,
where the applications are for breads. An earlier description of the Harvest King flour at the GM website gave the applications as soft rolls, croissants, puff pastries, and pita breads. Of course, this hasnít stopped many of our members from using the flour to make pizza, with what appears to be good results. Some members even prefer it over the King Arthur bread flour.
I donít want to rule out the possibility of the flour being a bad bag. Another member (Glutenboy) also felt that he may have had a bad bag, as he reported at http://www.pizzamaking.com/forum/index.php/topic,4857.msg41155/topicseen.html#msg41155.
EDIT (4/15/14): For a current link to the Harvest King flour, see http://professionalbakingsolutions.com/flour/brand/general-mills-harvest-king