There are lots of reasons for miscarriage. And most of them can't be helped. I am very sorry for your loss. This is a very private grief, I know, but please know that you have support.
We were told to wait at least one normal menstrual cycle before trying to conceive again, because that is a good sign that the uterus has healed, and all of the tissue has definately been expelled. In our case, the baby was lost because of a chromosomal defect. I imagine there are different waiting periods, depending on the cause of the miscarriage. For example, one lady who answered your post, mentioned a uterine infection - that would make sense to have a different healing/waiting time. If you haven't conceived yet, it might just be that your body isn't ready to. We forget, that pregnancy is really h*** o* the body. Nutritional stores start to be depleted right away. Depending on how old your baby was, your body might want some extra time to heal, and to rebuild those nutritional stores. Some folks who lost their babies very early, might have needed hardly any healing time at all, and conceived again right away. Don't beat yourself up by comparing yourself to others who had different successes. Every situation is unique and different, every baby is different, every mom is different.
Our baby was only ten weeks, but based on the size, the sonogram tech suspected that it probably died at eight weeks. It was lost in late June, my now almost-20-month old was conceived in late-August. By the way - my other daughter is four. There is a 3 year and one month gap between them, and they get along just fine.