That sounds very common for our lower mainland snail keepers even though they're dosing extra calcium. Our snail keeper friends back east that has higher mineral content in their tap water and they don't have the same issues. I suspect they have a higher natural magnesium content in their water supply. It could be your source of calcium or if you're just only dosing calcium for GH, try adding Epsom salt for Magnesium. My understanding is that magnesium helps with calcium absorbtion, maybe a 3:1 ratio calcium/magnesium? I'd be interested to see if that works for you.
The ramshorns I keep with a layer of Caribsea Florida Crushed coral with aragonite has nicer solid red shells than the ramshorns in my regular ramshorns in the community tanks w/o the natural buffering of aragonite.