How are you "assigning" material?
In part, assembly?
Attach the file please.
Or some screen shot.
What is the density of the material? Sometimes materials are so close in density it really does not matter the material type for the weight. Before materials came out I had set the density of my parts to carbon steel and that worked great for my stainless parts as well. If I used something like aluminum I would have to go and change the density setting of the part to get an accurate weight.
I think you assign the appearance, not the material.
Under Mass Properties, click "Override Mass Properties..." and make sure they are not manually being set as this would override any material you apply to the part.
Also where are you getting the mass? Mass properties or something that is showing up on a drawing?
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