Context: Planning in software projects is a difficult problem due to the uncertainty associated with it. There are many factors that cause difficulty in formulating a plan. Not many factors that influence the efficiency of planning are identified in the previous studies. The literature focuses only on technical aspects such as requirements selection and estimation in order to plan a release or iteration. Objectives. The objective of this study is to identify factors that affect planning efficiency. The context in which the objective is achieved is large scale complex projects that are distributed across multiple teams, in multiple global sites. The motivation for selecting large scale context is because most of the existing releases planning approaches discussed in the literature were investigated in small scale projects. Hence this context will allow studying the planning process in large scale industry. Methods. A case study was conducted at Siemens’ Development Centre in Bangalore, India. A total of 15 interviews were conducted to investigate the planning process adopted by Siemens. To achieve triangulation, process documents such as release planning documents are studied and direct observation of the planning meeting is performed. Therefore multiple sources are used to collect evidences. Results. The identified challenges are grouped into technical and non-technical category. In total 9 technical factors and 11 non-technical factors are identified. The identified factors are also classified based on the context in which they affect the planning. In addition 6 effects of the factors are identified and improvements perceived by the participants are discussed in this study.