TTC baby #3 but it's not happening

I have two girls 8 and 5, we are ready to have our last baby, but it's not happening, in July I had a missed miscarriage and had s d&c and right now I had what I thought was a BFP but had my hcg levels checked Wednesday and they were 20 today went down to 6....,I haven't started bleeding so I'm waiting to see how long before I start bleeding! Any recommendations? Should I start trying right away?