Most likely this is caused by the "new" formulation of Cabot stains. All manufactures are now going green and coming out with zero VOC products. Results will be mixed for the next few years until all of this is figured out.
For now you need to start over by getting rid of all products you used in the past and expose good clean wood. Sanding is the only way to do this right. Bleaching products and pressure washing can be used for to remove most of the mildew. Sanding will take care of the rest.