Ovulation and implantation

So I've been reading up on all kinds of information just so I can be educated while TTC. I've been reading about implantation lately. They say implantation happens on average 6 to 12 days after ovulation/fertilization. So if someone doesn't implant until the 12th day after ovulation, and based on general HCG rules, it calculates out that they wouldn't get positive if they are pregnant until possibly 16 to 19 days after ovulation? Or maybe longer depending on how fast HCG levels rise. What do y'all think? Anybody have experience with this or have it happen to them? Anyone not get a positive until a week or more after missed period?