I have a part with alot of surfaces. When i attempt to thicken the part to the inside it will thicken 0.84mm but no less. it will thicken any larger amount but not any thinner.
I've run into a similar issue before, but I don't have any solution for it. I quickly made a surface body to test some ideas on this issue, but I was unable to figure anything concrete out. If you could describe your surface part, then I might be able to help you figure it out. When I did run into something in my quick test, it appeared that one of the surface portions had folded over on itself in a few locations which made the thicken command fail. Aside from that, I don't have much to tell you.
I've had that happen, too. I think it normally has something to do with whether a surface can be properly offset by the distance you specified--and thin is sometimes a problem. Thick will eliminate small-radius curvature surfaces completely, and therefore eliminate the problem of offsetting such a surface.
So I think it's ultimately a problem of having small radius curvature in your surface. Small offsets try to offset too small a radius, whereas larger offsets bypass this problem (in a matter of thinking) by going past the zero-radius offset and regenerating the surface beyond the small-radius curvature.