What was the distance to subject? It could be a depth of focus issue, but you should have had a couple inches of DOF with that lens @ f/2.8 I also note you are using ISO 3200, that is not going to be the best for sharp images and if you use noise reduction software it will also soften the image. DOF calculator is here:
Online Depth of Field Calculator
When I have a problem I always go back to basics. Which means remove as many variables as you can. Use a subject that will not move, use very good light so you can get the ISO down to 100, camera on a tripod if possible. If you can get good images doing that, then start adding back in things until you find the problem.