To begin with the model needs to be solid, which means it must be component / group and water tight. Once you made that check windows- enmity info, if it reports volume it is solid. There are two plugins you should get and use, ThomThoms soild inspector and TIGS solid solver. The former will ID problem areas, the latter will try and correct model. You need all geometry to be one component ( no nesting), no text, 2 manifold.
TIG gave a good summary of requirements here Can we convert a sketch up model to a 3d printing model?
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