According me any project would commonly face any obstacle because of some of these reasons:1) Poorly defined goals, for this you need to use a planning software for making goals more clear. 2)Unrealistic deadlines, for this you need to consider many factors before making deadlines and have proper communication. 3)Scope Creep, for this you need good understanding of customer needs. 4)Insufficient team skills, for this document the core skills needed and train the team accordingly. 5)Risk Management, for this you need to predict every potential risk with strategic planning. 6)NOt using Project Management software, keep information under one roof. 7) internal issues in team, for this you need to do some team building activities among them.

So, according to me the planning and executing phase is the most important phase.