Welcome to the forum Bill. To your question, having IS on may help but you also need to consider other factors. Something as simple as hanging a weight under the tripod will help (try using your camera bag), don't have the top column fully extended, check your Aperture and Shutter settings, for example are you working with a shallow depth of field? Also, if it was windy was your shutter speed fast enough - your subject may have moved in the wind. Finally try using the hyper-focal distance for focusing ( focus on something 1/3 the way up from the bottom of your screen). Would love to hear if any of this was helpful.