Hi, Well the key is sunshine. If that one side of the arbor is receiving less sun than the other, it's likely the shadier side will never fill in very well. These plants need 6 to 8 hours of sun a day. They also like to have their roots cool which means to keep them mulched. There's an old saying that clematis like their heads in the sun and their feet in the shade. Otherwise, there's not much you need to do with them. You can apply a little slow-release granular fertilizer at their base, but basically that's about it.