[Ol` Joe]

The best way to tell if your cases need trimming is to measure them and quit guessing.
I agree Though, it does sound like you need to size your cases a bit more. The directions with most dies tell you to raise the press ram and screw the die down until contact is made with the shell holder and give them a 1/16 -1/8 turn more, allowing the ram to "cam over" when sizeing. I`d try this set up and see if the case chambers. You can unscrew the die a 8th turn at a time checking the chambering until you reach the min sizeing that will allow your cases to chamber. The die does most of it`s sizeing in the last couple of hundreths of seated depth, remember the shoulders only need to be moved .010" max in most cases from min headspace to max. (+.006 / -.002 is a common headspace tolerance)
The other post recommending lubeing the inside of your case mouths is a very good point. I`ve seen unlubed cases have the expander pull any sizeing done to the shoulder right back out as you open the mouth up. Definately take a Q- tip or nylon brush with a "VERY LITTLE" bit of lube on it and wipe the inter mouth of your cases. This may be all the change in your present method of sizeing that you need to do. Again very little shoulder excess will cause a tight fit in your chamber.