Have you made the .col (collision) file?
If you're doing it for SA then here:
If you have Kam's scripts, go to Utilities panel -> Collision IO, then a box will appear. Click on 'Collision Mesh' button then select your object. [COL3 button must be selected if you're exporting it for SA] After that, write the name of the object (it has to be same as DFF name) in a box above and then click "Export" button. You will get a COL file you need to add to gta3.img (same archive where are .txd and .dff file of the object)
Once again, COL name must be same as DFF name.
Also, the object isn't .dff object lol. DFF defines its shape and textures, TXD stores its textures and COL defines its bounds, so eg. you can't go through it ingame.
This post has been edited by AB0333 on Saturday, Feb 25 2012, 18:27