Hello, Spikeyhedgehog. Welcome to WAB!
The individual flower spikes on buddleja don't last for that long before they turn brown, so you probably don't have a problem there.
If you want to increase the number of flowers then follow the pruning advice given by Darley, and try cutting half the branches back at the beginning of March and the other half back at the beginning of April - that way you extend the flowering period.