By some strange coincidence it seems that in my current game often when I marriage to pull off an advantageous matrilineal marriage the couple tends to stay childless. Is there actually a hidden modifier for fertility in such circumstances?
Only your cognitive bias, rest assured. There are examples of the opposite scenario happening (celibate people having children), so it is possible that you just have an unlikely streak of bad luck.
Could be your court clogging up and limiting fertility, even though it makes no sense, as at that stage of overabundance it'll be nigh impossible to get rid of them.
You can land the husband, provided he has no other children.
Dumb question, but what was her age at the start of the marriage?