Plant growth depends on the soil the plants are in.
What is the soils pH?
What nutrients are in that soil?
How well does that soil drain?
How well does that soil retain moisture?
What kind of life is in that soil?

all that said.. when did you plant it ...

if newly planted.. it is doing all its work underground.. growing a rather massive root system.. to support NEXT YEAR SHOW ...

perennials dont grow as fast as annuals.. so you have to think about why.. and that is when i think of the root mass ...

its only failure might be its inability to meet your expectations ... its all about delayed gratification with some of them ...

and at that point.. you are relying of genetics ... you can not 'make it' do it any faster .... so dont think if you hyper-feed it.. you can make it all happen faster ....

ken

Are they small plants often sold at big box stores in a plastic bag with peat moss and a picture card stabled on the bag ? If so most find those do better if planted in 1 or 2 gallon size pots and grown on for a year or two before planting in the garden..because they are so young and it's easier to keep an eye on them that way. If this is the case, don't worry, time will fix things.

What variety are these ? Some are just smaller plants or slower starters. Though that trellis looks too small for two clems, in most examples. Time will tell.

You can look up the variety and double check that you have these placed optimally in regards to sun or partial shade requirements.

If that's full sun it would be better to shade the roots. I would move a small bunch of potted plants over around the front of them considering the time of year now is not good to be transplanting. That should cool the root zone pretty well.

And sometimes one variety will just mysteriously not live up to early expectations. I have one like that now. My first hint should have been the complete lack of other gardeners reviews on this newer variety, C. Yukikomachi. Purchased last year as a two or three year old plant, babied exactly the same as several other new clems.Cut back to two nodes early this spring as is recommended for all new clems to create a better formed, healhier plant in the big picture.

All the other new ones have gone nuts climbing and blooming. This one sits, a leaf or two now and then. Now it has a couple tiny little buds forming. Pitiful I tell you. At least I know it's nothing I'm doing wrong, though I'm starting to feel like going out there and explaining what happens to 'bad' plants to it, as I show it the yard debris can. Think I could 'scare it straight' ?

This post was edited by plaidbird on Fri, Jun 28, 13 at 13:39

Clematis have rather specific growing requirements to flourish -
1) a deep and enriched planting hole. Although it is seldom recommended for other plant types, clematis appreciate a large and well-amended planting hole. Typically the recommendation is for an 18"x18" hole amended about 35-50% with organic matter like compost. Good drainage is critical and can be compromised when adding a lot of organics so make sure the site drains extremely well initially.
2). Clematis appreciate being planted deeply. Again, contrary to recommendations for most other plants, clems should be planted 4-6" deeper than the level in the nursery container. This encourages additional stem formation from the root crown and healthy root development, both of which help with establishment and resistance to clematis wilt. As noted, if purchased bare root or in tiny sizes, growing on in a container for the season can be very beneficial. You want the roots to at least fill a 1G container before planting out.
3). Shaded roots are a bit of a myth :-) What the vine wants is a cool and evenly moist root run. While shading can accomplish this, attention to watering is far more important. Also mulching or underplanting with a spreading shallow rooted groundcover or perennial is helpful.
4) Clematis should be pruned back hard (to 6") for at least their first two seasons in the ground in late winter. Fertilize well with a tomato or rose fertilizer after planting and again once or twice during the growing season.
5) Finally, clematis take from 3-5 years after planting to establish fully. There is a common saying "the first year they sleep, the second year they creep and the third year they leap" :-)