Without any photos, nearly impossible to say. Have they received the proper fertilization? Large enough containers to accommodate proper root growth?

I would be less concerned about any that are newly planted. It can often take several seasons before new clems are sufficiently established to bloom well, if at all.

The ones that have been in containers for several years, what did you use for planting medium and has it been changed out at all? Typical bagged potting soil breaks down and becomes compact over time. Has the moisture in the pots remained even, not too dry or really soggy?

I use Miracle-Gro Potting Mix, amended with Alfalfa Meal, and Rose-Tone. The moisture has been consistent - but even with that I have about 5 Clematis that are being discarded, they are nonproductive and have not flowered.

Miracle Gro potting soil has a very widespread reputation for not holding up well long term as well as being excessively moisture retentive. Most container gardeners avoid it. And the dry organics are not well suited for container culture. They require the activities of high populations of soil organisms to break them down into plant usable forms and these are just not present in container soils. You would be much better served with a fully water soluble or liquid synthetic fert.

In terms of potting soils, there is nothing else other than Miracle Gro Potting Soil in Garden Centers - So I don't really know where you would go to find a different brand of potting soil. Miracle Gro products have taken over the market .

How big are these plants? We’re they tiny roots from a bag or large plants from a nursery?

There's a number of other better brands out there...they may just require some hunting. ProMix is pretty good as is Fox Farms - their Ocean Forest potting mix is one of the best. These are carried by various garden centers in New Jersey. And Kellogg's soil products are carried by most of the Home Depots in the area and they are also a superior brand.

Also, a great many container gardeners make their own mix. The Container Gardening forum is full of discussions on this topic. And if you were to do so as well, that would be the preferred method by far.

All of my Clematis were purchased from Clematis Nurseries.

I once lost over a dozen tropical plants that I'd had for years over winter in my greenhouse due to Miracle Gro potting soil. It does not drain well. I normally use a local brand and add perlite but for some reason I didn't and lost gardenias, hibiscus, fuchsias and more. If it's all you can get, you need to amend it with perlite or vermiculite at the very least. Its great for summer annuals in hanging baskets since it holds water but not for shrubs, perennials or vines you want to keep in pots long term.

Well that is probably why I lost 6 Clematis plants this year that were planted in pots and Urns, and a few of them were brand new plants that I just purchased this Spring. They all got wilt and that was the end of the plants. Many people have told me not to use Miracle Gro. I'm going to have to look for other brands of Potting soil in my area. Thank You.

Wilt is seldom the "end of the plant". Most vines that contract clematis wilt will come back from it rather handily, if not necessarily rapidly. Salvaged nursery returns due to 'clematis wilt' accounted for a heavy number of the vines I grew in my previous garden :-) Just cutting back and little TLC for the rest of the season and most reappeared unblemished the following year!! The disease is seldom fatal.

OTOH, root rots - what can occur in an excessively water retentive potting soil like the MG - most often are fatal.

All (6) clematis plants of mine grew well - but ended up having nothing but vines and no buds or flowers .... then the leaves began to turn black. Yes they were all in pots and containers and yes I did use MG. Now I know not to buy MG and to look for some other brand of potting soil. Right now, all 6 plants have been cut back to ground level.

Good....at least you didn't toss them!! If it was wilt, they should come back. If they fail to reappear at all and the roots are soft and mushy when you do repot, then the culprit was most likely a root rot.

How long will it take for them to come back ?

It varies. Sometimes you will see new growth from the base in just a few weeks. Sometimes not until next spring. And less frequently, it can take several seasons/years before you see regrowth.

I do believe one problem was that I planted the new plants in containers that were to small, to shallow - Big Mistake! I believe that also caused the plants not to grow well. I will leave the pots on my patio and wait and see if they do anything .....

Since the current pot size and soil mix was at least one of the possible causes of the problem, I wouldn’t leave them in the same conditions and expect them to come back.

I agree with Babs. I somehow got the impression that you would be repotting in a better soil mix right away. If that is not the case, then I would encourage you to do so. Most of my containerized clems are in pretty large containers.....at least 7G and a few in 10 or 15G containers. That seems to accommodate them well for at least 3-5 years before I need to repot and refresh with new soil.

Well I was able to locate who sells the Potting Soils that were recommended by gardengal ---- Pro-Mix is sold by Amazon, so is Ocean Forest potting soil. Kellogg's is sold locally by Home Depot.