Hello plumbing pros! I'm nearly done with my bathroom refurb, but got a problem fitting a shower arm I bought from soak.com. In the attached pic, the chrome thread is the one that screws into the wall. The brass thread is for a handheld shower fitting from the same set (for comparison). I can only get the shower arm thread to go about 1/2 turn into the backplate elbow. You can see the shower arm thread is flatter, unlike the sharp brass one you would typically see. The brass thread screws in beautifully. To my untrained eye these all appear to be BSP(P). So the logical answer to me is that the chrome thread is not cut properly. I asked soak.com to investigate and they sent me an identical replacement. What do you think guys, am I missing something? Or is the shower arm not fit for purpose?