Sulcata Tortoise with VERY soft shell HELP !?

I have 2 sulcata tortoises, about a month or 2 ago I noticed they were getting alittle soft, so I bought so calcium. One of the tortoises is fine now, nice and firm shell. The other one . . .has gotten worse !!! The shell is sooo soft on the sides and the underside !! I sprinkle the calcium on his food, and I give him time outside so I dont know why he wont firm up !! What can I do to fix this ? They are 7 months old. I feed them Grass, Romaine lettuce and rose petals.