After a miscarriage it can take time for your body to get back to normal. I've had this same thing happen a few times. Best thing to do is just keep testing. You might get your peak tomorrow or only in another 2-3 weeks. I would also test twice a day and not just once just to make sure you don't miss it.
Your hormones won't settle into place for a few more weeks... Try testing again after you get your period. Unless you have PCOS - in that case, don't waste your money because they won't work right.
In my experience it's very easy to miss the HCG peak. I would forget about them and have sex every 2-3 days since day 7-8 of your cycle depending on your cycle length