Your tests are definitely darker, and the fact that those are all in the span of 24 hours (when FMU is supposed to be the strongest), is a really good indication and sign!!
I know it must be so stressful, but I would really see if you could get in on Thursday. That would already give a good idea of the doubling rate. Also remember, some people have a slow rising HCG and it doesn't have to exactly double. My numbers didn't double at first. What matters is that they rise and the doubling is supposed to be 48 - 72 hours.
Found this online:
* At 14 DPO, the average HCG level is 48 mIU/ml, with a typical range of 17-119 mIU/ml.
* At 15 DPO, the average HCG level is 59 mIU/ml, with a typical range of 17-147 mIU/ml.
* At 16 DPO, the average HCG level is 95 mIU/ml, with a typical range of 33-223 mIU/ml.
* At 17 DPO, the average HCG level is 132 mIU/ml, with a typical range of 17-429 mIU/ml.
* At 18 DPO, the average HCG level is 292 mIU/ml, with a typical range of 70-758 mIU/ml.
* At 19 DPO, the average HCG level is 303 mIU/ml, with a typical range of 111-514 mIU/ml.
* At 20 DPO, the average HCG level is 522 mIU/ml, with a typical range of 135-1690 mIU/ml.
* At 21 DPO, the average HCG level is 1061 mIU/ml, with a typical range of 324-4130 mIU/ml.
* At 22 DPO, the average HCG level is 1287 mIU/ml, with a typical range of 185-3279 mIU/ml.
* At 23 DPO, the average HCG level is 2034 mIU/ml, with a typical range of 506-4660 mIU/ml.
* At 24 DPO, the average HCG level is 2637 mIU/ml, with a typical range of 540-10,000 mIU/ml.
So according to this, anything between 17 and 119 is in normal range. And remember, there is no clear way for you to know that you are exactly 15dpo. Even if you ovulated exactly on time, implantation could've happened any stage.
You are definitely still in there. Prayers are coming your way all day hun!!