This card is weird, sometimes, when your opponent activates a normal/flip monster effect Solid Dragon doesn't ask you to activate his shuffle effect.
This problem doesn't occur when it is an effect that targets Solid Dragon however.
If I'm not wrong, I think when Red eyes baby dragon or when apprentice illusion magician activates any effects Solid dragon cannot shuffle.
I have to verify that.
When effect's like Gravekeeper's Descendant's targets you however there is no issue