Last week, I submitted a similar problem and got some great advise from a member of this group. However, I spent the weekend re-modelling and now have the same problem again. The attached part should have the whole inside face shelled to 1.5mm. I can only get the 2 arms to shell and that required 2 steps for some reason.
I've also tried deleting the inside and side faces and thickening to 1.5mm but that also fails. This method is suppressed in the model so if you want to look at that way, just suppress the 2 shell steps and unsupress the delete face and thicken steps.
Any help is appreciated - am under tight deadline as usual.