If problems come up, creators are expected to post a project update explaining the situation. Sharing the story, speed bumps and all, is crucial. Most backers support projects because they want to see something happen and they'd like to be a part of it. Creators who are honest and transparent will usually find backers to be understanding.
It's not uncommon for things to take longer than expected. Sometimes the execution of the project proves more difficult than the creator had anticipated. If a creator is making a good faith effort to complete their project and is transparent about it, backers should do their best to be patient and understanding while demanding continued accountability from the creator.