The seat is too detailed, with too many polygons, and however good Archicad is in showing complex geometry, an area can be too much.
Try using the inbuilt theatre/stadium seating Archicad object (not cell, that might be the Microstation term?) for general layouting, and you can use the highly detailed RFA model for visualization close ups.
All this is in general because the problem is not easy to understand the way you have explained it, and this is my best guess...
... once upon a time, about 20 years ago, I wrote a small HowTo on converting the 3ds/DWG/DXF meshes to GDL on an example of a Mercedes GTR race car. The principle stays the same: take care of your polygons!!!!
To make sure it is the polygons, and not something nastier: Select that object in 3D window, and run PolyCount. Then you shall see.