Elderberry not actually having any berries

I have an elderberry (Sambucus canadensis) that has been producing flowers for the second year, but very few of the flowers are turning into berries.

I'm trying to determine what the problem is:

Option a) Elderberries don't self pollinate very well and thus I'd need a second bush?

Option b) Elderberries should self-pollinate, and I just need to attract more pollinators?

Option c) It's something temperature related. The flowers did not like the intense heat we had this year?

Option d) It's just normal for only 10-20% of the elderberry flowers to turn into berries.

I am open to getting a second elderberry, but it's a space commitment so I'd like to know if that's the right problem. And I'm finding contradictory information on the web.

This:

"Option a) Elderberries don't self pollinate very well and thus I'd need a second bush?"

You can easily make new plants from cuttings.

Plants from cuttings are the same individual so won't serve as a pollinator.  Cuttings need to be from a different individual plant.

I've noticed lots of wild elderberry growing nearby, especially near a nearby stream. If you can take a cutting of a wild elderberry in the Fall or Spring when its dormant, and stick it in the ground, you'll probably have a good elderberry pollinator for your other one before too long!

I have had great success with the Bob Gordon variety from River Hills Harvest.
Thank you for all the great replies!

Thanks! I feel alot more confident getting a second plant, knowing the issue is cross-pollination.

Update two years later: my elderberry started fruiting on its own laat year, and this year looks like a bumper crop.

The first flowers of the seasin have very few viable berries, but the later are almost all fertilized. So it turns out my elderberry was a decent self-polliniser, but needed a few years to mature or more visitors.

Adding plenty of flowers all around probably helped too.