One of the secrets for Clematis is 'feed, feed, feed'. So get the watering can out and have a go! Also, depending on the type of Clematis, you may have missed out for this year. Do you know what variety it is? (Answer below, for the reply to get on the same page as the question).

You should also find out the name and the pruning type of your plant; there are 3 pruning "codes" and quite possibly you have pruned a spring flower which only requires pruning lightly after flowering.Pruning this type at any other time prunes off all the flowers.

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Agree with Alysoun above, depends which variety of clematis and whether you pruned it - if you bought it this year, you may even have bought a variety that flowers in the spring, which would explain why you have no flowers.