Maybe you have non-manifold geometry, are the normals all pointing the same way?.
In the convert settings there is a setting that's called maximum base mesh faces, it's stands on 1000 faces as a default. Look at the polygoncount of your model and adjust the number in the convertoptionbox to that and add 500 poly's to be on the save side.
To find the polycountdisplay, goto display, heads up display==> polycount.
select the combined model and look at the number of the faces

If this doesn't work post a snapshot of your workspace so we can possibly see what's wrong

Good luck