Have you been pruning or cutting them, Ann, and if you have, when, as in what time of year? And are they in pots or in the ground?

Yes, prune them at this time every year and they are in the ground but have never fed them, could that be it. I DO prune them back quite hard, would that cause them not to flower?

I imagine you're not sure of the varietal name of the clematis you have, but the only ones which should be cut hard, preferably in midwinter, are the viticella types, which start flowering early July, sometimes June, and go on till late September/early October. All the others don't really need pruning, so it might be you've been cutting off the growth that would have flowered the following year.

Frequent feeding with high nitrogen products can also interfere with flowering.

Thanks, I do have a fourth plant which is flowering profusely as I speak so the other three should have bloomed by now and I haven't fed any of them at all. Maybe I should?

No, too late this year. They don't need much if they're in the ground, a top dressing of fish blood and bone or Growmore or Osmocote in spring, turned lightly into the soil, will be sufficient. They do benefit from a mulch around the base in spring, after you've put the feed down, though, specially of something humus rich such as good garden compost or well composted animal manure. As its not a feed problem, the likely explanation for lack of flowering is incorrect pruning.

OK Bamboo, many thanks for that - I'll leave the pruning this year until midwinter as you say and try a feed in Spring, keep your fingers crossed for me. Cheers

I'd leave them alone entirely - without knowing what variety they are, they may not be ones that need cutting at all. The Prune group 2 varieties all flower on the previous year's wood with large flowers around May/June, and then again later with smaller flowers. Cut those and you end up losing the large, early flowers. There's a spring flowering group which shouldn't be cut as well.
When they used to flower, can you remember what time of year it was? Have they only refused to flower since you started cutting them back?