I often hear technical people say: “my customer won’t make a decision and therefore I can’t begin work.”
Our job is to make it easy for our customers and stakeholders to make decisions. Try asking them questions to figure out where they are stuck. Can you prototype an interface so that they have something to work with (or against)? Do they understand their own business rules? If not, can you help them to document the important processes?
In short, don’t just sit there waiting, take action to help both you and your customer move the project forward.