i wonder and theorise that this is actually due to being your protectorate.

im my Vh/Vh Poland game i gifted my protectorate saxony massive amounts of land till they owned most of central europe, despite this and despite having Massive armies sitting bout they wont attack any1 even when i bring them into alliance wars. its quite odd. they had like 2 stacks sitting next to a french town when i declared and brought them in and they did nothing.

Just my 2 Cents

cheers knoddy