Early miscarriage due to previous birth?

Hi!
I have a question. 4 months ago I gave birth way too early to my late son. We both wanted to move on and try again quickly. We waited 2 months to see how my body would react. It actually got its normal cycle back right away. And everything looked fine at the follow-up check-up too.
I was pregnant 4 times before, and with all 4 pregnancies I got pregnant on the first try. But now it's happened twice that I get a positive test around my expected date, and then 2 days later I get my period. After giving birth to my son, the placenta didn't come out, so they removed it in the OR. I don't know exactly how, but she said it's not the same as a curettage.
I was wondering, could that be why implantation isn't going well? Or is it something else? Or am I just unlucky this time?
Love