backnut

Hello all. I am trying to install a radiator and I am stuck in the last step. So, for whatever reason, I can't tighten the backnut with the olive nut and the lockshield valve. I can't figure out why this is happening. Any thoughts? Thanks in advance!

Hi all, Need to check I am not being dim - I have had a Todays Tools TT Professional 1/2"-3/4" Tap Back Nut & 15-22mm Compression Fitting Set lying around for ages (one of those things that someone else was clearing out and I thought might be useful someday), however, when it comes to changing...