Another possibility might come from the number of embryos they transfer. In most australian clinics they now transfer one as a standard procedure, but they might do two after some discussion of the risks/benefits (ie, greater risk of twins/triplets, which is a higher risk pregnancy.)

Also, has she been fully and closely monitored during the luteal phase, ie, monitoring of progesterone levels and so on, because she might have hormonal deficiencies which were overlooked when they found the MFI - ie, they find poor sperm, and don't look any further for reasons that they're not falling pg naturally. If she did have hormone level problems, this might explain the repeated failures, and they need to modify her luteal phase support at that stage.