Where are they planted? It's hard not to succeed with them so it's likely a climate, siting, or culture issue at play.

They are planted where they get morning sun. They are fairly close to the house and get very little water during the summer. Thanks for your help.

They are planted where they get morning sun. They are fairly close to the house and get very little water during the summer.
I'm guessing that like me, almost all of your rain comes from the West. Since they are close to the house, as you say, I'm betting they are not getting near enough moisture throughout the whole year.
If you are intent on them multiplying, I'd suggest you moving them away out some from the side of the house, planting them in another location, or else watering them fairly regularly, at least during their growing season and as they head into summer dormancy.

Thank you, chemocurl. I will replant them farther from the house and make sure they get watered.