I'm dealing with recurrent miscarriages as well. The ran a ton of tests on me. The main know causes are genetic/chromosomal (>5%), hormonal abnormalities (like LP deficiency), metabolic abnormalities (ie diabetes), Uterine abnormalities (10%, but I doubt this is an issue with most of us experiencing secondary infertility), Antiphospholid syndrome (3 - 15%), thrombophilias, and male factor issues. 50 - 75% of the time it is unexplained. (this is from a little info sheet my RE gave me)
I'm sorry you are experiencing this. I know how tough it is.