Problem when importing an Autocad 3D assembly to Inventor

Hi guys,
I received an Autocad 3D model of an assembly which I need to import to inventor. I select Open-Options-Import...etc and go through the import windows and when the models opens in the Inventor's .iam environment, several parts are not where they suppose to be, and I have to manually constraint them again into the correct position.
Any ideas on what can I do about it?? Any tips to improve the import process will be greatly appreciated!!
Thanks in advance!!
MECA