I've given lots of referrals to professionals in the past who rarely reciprocated. I got so used to it that I never bothered to dwell on it.
Those referrals of yours, Peter, amounted to some big commissions. I would simply cut off the referral relationship pronto. You could say something to her if you like, but I wouldn't bother. I'd just cut off the referrals.
If someone does not pay attention to the important but discretionary details like "thank yous" then I would question that person's devotion to other details as well.
It's a worrisome sign for sure.
If someone gave me a referral which resulted in a big sale, I'd take them out to dinner at least!
Give her the big boot!