On both my partly built (by someone else) reference kit and unbuilt kits, the bottom door opening area attached to the cowl appear to be slightly warped, probably by the body's molded-on sprue brace that's supposed to prevent that. Most obvious on the right side.
I figured it was something like your problem that inspired the builder of my reference build to glue the doors shut, as well as the guy in the step-by-step Korean build journal on the CD. Martin Swire, however, seems to have had no problem, though he said he had to add pins to a lot of butt joints elsewhere. But he built his kit many years ago.
http://www.freewebs.com/martsmodels/1928mercedesss.htm