A good way to find out if you have an open quilt (which would prevent you from being able to solidify) is to look at the model in wireframe. If the an edge is purple, it is bounded on both sides by quilt patches (which is ok) and if it is pink, it is a one-sided quilt edge and is most likely the source of your problem. If you have multiple quilts in your model you may want to hide everything except the one you are solidifying.