This paper identifies and analyzes barriers startups encounter when following the Lean Startup approach, i.e. the Customer Development process during the initial phase of identifying and validating a business model. In an action research setting, we mentored and interviewed eleven startups enrolled in a business development program called “Born Global” running 7-10 months. For the barrier prior experience, entrepreneurs who had previously encountered failure, had a long period of unsuccessful work, or had heard about Customer Development previously were more likely to follow the process. A main finding was that the entrepreneurs were busy and lacked time and/or money. Deeper reasons included team conflicts, lock-in to an earlier business model by means of a business plan and boards intent of following the old business plan. The lock-in to the old business model can be explained that several startups had gotten a first round of funding based on a flawed business plan.