First Response vs First Signal?


So I took a first response on Christmas morning (roughly 8-9 dpo) and it tested was VERY faint but definitely there (I confirmed with a few other people and they also saw the very faint pink line). I'm about 3 weeks and a day. I tried testing later Christmas at night AND the next day with one of those cheap Walmart First Signal tests and nothing showed! Does anybody know the sensitivity difference? I'm worried now the First Response was faulty because I havent been able to get a + on the First Signal but I know theyre also a lot cheaper and maybe I just done have enough HCG for that cheaper test yet? any thoughts? or similar experiences?