I wanted to add to the Mumsnet hive mind/info bank - leaving this here for anyone obsessively googling in the future. I am a POAS addict and bought WONFDO tests from Amazon. The Wondfo picked up the pregnancy earlier than FRER (at 9DPO). The tests got progressively darker up to 5 weeks pregnant using FMU - with the test line darker than the control. Then at 6 weeks and 6+1 and 6+2 the tests suddenly got lighter, with the test line back to being noticeably lighter than the control line. Having experienced 2 prior miscarriages I assumed this mean HCG dropping and pregnancy not progressing. Google told me it might be the Hook Effect, where some (often internet cheapie) tests struggle to pick up higher levels. To continue to torture myself I tested again with mid-morning diluted urine and the test came back with very dark test line, much darker than the control line. Three hours (and several cups of tea) between tests and a very different result.
I just wanted to leave this here for anyone in the horrid spiral of thinking a later test (from 5-6 weeks onwards) is showing bad news, it's worth a try. And of course, we all know its far better not to test once you get a positive result etc etc (but I am guessing there are others out there like me who don't have that kind of iron will!). Love and empathy to everyone out there on the emotionally exhausting conception and pregnancy journey.