What is THE biggest challenge when it comes to realising automation projects?

Please answer in as much detail as possible. The more information you give us, the better we can tailor a solution. 

Avoid answering with imprecise terms such as "requirements" or "programming" and try to provide more detailed information, e.g. "Constantly changing customer requirements make it difficult for me to submit a final project offer" or "Since we can't find programmers, projects are always delayed".