I've never had a problem with the Walmart brand. I use it mostly in UDS and RO lump in my other cookers. You may have just gotten a bad bag. The walmarts around here will sometimes store it outside in the garden section. I do not get it from outside in fear of it having moisture. I always look at the bag too making sure the colors of the bag look bright and new. Never had issues when do that.
My arsenal, Stumps Clone, Two Modified offsets, open pit with grate and rotisserie,UDS, and a free to me Weber One Touch Silver 18.5" kettle