Was it discussed at start of contract about expectations of progress velocity and cost? If it was and you met those, they may have changed their minds about tha

Was it discussed at start of contract about expectations of progress velocity and cost? If it was and you met those, they may have changed their minds about that and not much you can do to change that they did that. If it wasn't discussed, in the future with new clients, discussing expectations around speed of feature delivery and associated cost could help you filter clients.