Can you post photos, Peter? Since a great many commonly grow clematis are hybrids, it is possible that they have reverted to one of the parent species but it would be pretty unlikely for them all to do so at the same time and also all have similar parentage. If we can see what they look like now, that may shed some additional light.

Pictures please.

An overall of the clems and their neighbors.

And close-ups of the vines that includes flowers.

Please look my profile for Pictures. Thank you.

I agree with GG48 both as to the ID of the clematis in the photos and that it may have simply grown large enough to engulf the other clems. C. montana can be very vigorous! It also blooms earlier than many other clematis, so you may find that the others show up later, that it's just currently the only one blooming. As GG suggested, additional photos of leaves taken closer up and in various places along the fence where you see plants sprouting from the ground may help since not all clematis have the same type of leaves, so we could at least see if some of the others survived.

Sorry but no doubt. All different plants bloom now and exactly in the same way. Last year one of six turned white. This year all the different plants. More pictures tomorrow - rainy day here in Aalborg, Denmark

New Photos added. All - different - plants are bloomin now - in White.

I think I can convince myself that I can see the foliage of some large flowered cultivars in amongst the C montana leaves. I agree that the montana has engulfed them. They may still produce some blooms later in the year. My C montana is past its best now but none of my large flowered hybrids or viticellas has started yet.

No new photos showing ...... :-)

I looked on the OP's profile page.

I also see leaves of the other clematis in some of the photos.

In photos 5, 7, and the final one in particular, I can see two clearly different types of leaves, one type that is narrower and toothed along the edges and a different type that is smoother and broader. So the C. montana has grown over the other clematis in its quest for more space. Whether the others will bloom this year or not, they won't survive too long with the montana overpowering them. They will be shaded out.

I agree with the above comments - the montana has just grown to the point where it has engulfed the other clems, which simply have not yet bloomed (or are too shaded out to produce much flowering). I would suggest cutting it back soon to allow the other types to flourish but that may not be an easy task in differentiating between two congested vines. This is one of the reasons it is seldom suggested to interplant early blooming or pruning group 1 clems with the later blooming hybrids - most are simply far too vigorous to be good companions and are not typically pruned at a similar time, if at all.

The alternative is to just let the montana do its thing and move the other clems to a spot where the montana isn't going to reach to. If pruned back a bit, watered well and given some rose fertilizer in the new spot, they should recover.