One aggressive male.
So I recently started a new dedicated shrimp tank, with about 10 female PFRs and about three males. For four months....no pregnancies at all. The females remained saddled but no berried ones. I got inpatient by that point and went and found one more male (males are hard to find at the LFS). And after this new males goes into my tank....three days later ALL my females are berried! He knocked up all my beeyotches!!!!!