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.
After my miscarriage it took 6/7 weeks or so to get my first period - it takes your body time to recover, especially depending on how far along you were when it happens. Take your time, don’t rush your body, it’s been through a lot too. I didn’t start doing ovulation tests until after my first period, and also used natural cycles to track my temps to give me an idea of when to start testing.
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