Are you taking any meds which could interfere with af arriving? Or possibly cause a line early on which wasn't genuine?
I have heard mixed things about Superdrug tests. Their reviews on their website aren't good, but I have heard many women praise them, so I think it's probably too individual to label them reliable or unreliable. Some women don't see positives with their fmu but will with smu or later during the day or evening, some don't see a positive urine test altogether which you have said you have a family history of. I think I would be demanding bloods at this point.
Also, looking at your chart you have a pretty significant dip at 12dpo which could be implantation, it can definitely happen later on, and some people don't see a positive until quite a few days past this. Fingers crossed for you