Is very normal to keep checking, my DD had DDH was in a brace, out of brace at 6 months, x-ray at 12 months and we have another one at 18 months, one of the reasons they keep checking is that it takes a while for bone to form, so with my DD at 12 months they thought the cartilage was all ok and hip in right position but not enough bone had formed yet to clearly see it all on the x-ray so they want to check again at 18 months. When she was first diagnosed I was told they would probably X-Ray every few years until she has finished growing.