There is always the possibility it was mislabeled, although dark purple June flowering clematis are rare.

Dp you prune the plant? If so, how? Any type of fertilizer? What is the soil like and amount of sun it receives?

Reply to gardengal48

I pruned the plant after the first year it flowered but not since. The old growth produces greenery the following year. When I did prune it, I pruned it about 6 inches from the base. The soil is a mixture of clay, to which I added generous amounts of sandy loam, peat moss, compost and a bit of well composted sheep manure. I add compost each year but rarely do I add any commercial fertilizer. Three pink spirea shade the roots. The area is protected and has a southern exposure. It begins to get shade at about 3 o'clock in the afternoon. The site can be dry since the clematis grows on a trellis against a white brick wall of the house under the eaves.

Jackmannii wants to be pruned hard like that every year. It blooms on new growth.

The spirea may be sucking moisture and nutrients from it, I would make sure it gets hand watered and composted/manured.

Clematis don't need to have their roots shaded, they need their roots moist. Shade usually means moist which is how the myth that they must be shaded got started.

Jackmanni even at it's best is not going to flower all summer long.