Some factions have a hard time. It's easy to have trouble finding husbands for your princesses as Spain due to a lack of provinces. I usually have one marry the unrelated general you're given. Venice can also have a hard time, as, if you aren't expanding you'll soon be overwhelmed with a flood of sons. But yes, your generals will refuse to have kids if you don't have enough provinces. It's really weird...

That's why it's best to keep the family streamlined, room to grow.