What is the most difficult part of any project to execute in time or what are the obstacles often arising mostly during or before project execution?

When time-managing a project we depend sometimes on individuals, departments or suppliers that do not directly ‘report to us’.
Therefore being able to influence those external stake holders is an important ability for any project manager and his/her timely completion of the project.

I understand that project stages can be divided in to four major categories.1.
Conceptual stage (which includes reconnaissance, feasibility and approximate budgets)2.
Detailed Study ( Detailed planning, designing and detailed budgeting of whole project)3.
Execution of Project.4.
Testing and commissioning of the Project.5.
Handing over the project to further / end user.
A good or excellent work and dedicated intentions towards project in previous stage eases the next stage of work.
Callous attitude and transferring attitude of problems to the next stage creates severe problems to the whole project.
So, picking up the suitable persons for each stage of a project without any preoccupied preferences is a big thing and solves many problems.
Start of each stage and end of it (handing over to further stage) are generally difficult parts.
Start involves too many constraints like acquaintance with details and environment, formation of cohesive team, convincing, etc.
Ending part also difficult, as it has to be handed over to other team.
it is a matter of detailed communication and trust and also outcome of earlier work.
Apart from these general difficulties, there could be specific bottle necks depending upon the nature and circumstances of the project.

Project stages are Initiating , Planning , Executing , Monitoring and Controlling & Closing.
The better you do your homework on the initiation & planning the better you will perform on the rest of the stages.
The most complicated part in the initiation stage is indentifying the stakeholders evaluate them and prepare your strategy to manage them.
On the planning stage the most complicated part is agreeing on the project scope.
The rest of the obstacles can most likely refer to communication.

I see that the obstacles arise before project execution is approving the project's plan and alocating the resurces required to complete it.

Obstacles which arise during project execution are mainly due to the rise of unforseen risk or the discovery of inadequate resource selection.

As per my opinion there are only5 major obstacles:1) lack of proper and detailed scope of work definition,-2) conflict between contractor and owner regarding contractual terms and conditions which is not being solved by mutual discussion,-3) inability to control labor costs,-4) lack of planning,-5) poor information management and poor communication amongst stakeholders,-

client approvals (sometimes including user acceptance) and stakeholders buy-in are the critical steps in projects, the amount of re-planning and alteration in the product / service you are intending to get is dependant on those, sometimes, you would face problems from within your project, not being able to perform tasks or activities in a certain planned way, or facing difficulties with vendors, materials, software or resources, these can kill your deadlines if you don't actively work on resolving them.

Since I have only worked at the project support level ...I would say you are right to bring the time factor in play.Project Planning is done on the basis of time allocation.But as i read the previous answers they were right to point out approvals and drafting/redrafting is also a mammoth task in itself.But the implementation phase would be critical as you have to ensure,making sure that all goes as per planning as it is the biggest critical factor in the project success.Obstacles can either arise in the pre implementation phase,during implementation or even post implementation since you can only close a project once engineer feedback/successful implementation/customer confirmation is received.....

Most Possible Obstacles Before or During Project Execution

Here are some that I have faced in my projects:

 

BEFORE: conflicts with stakeholders, scope definition 

 

DURING: stakeholders expectation, delay/ non-compliance by implementers - usually missing promised deadlines or quality, act of God/ unforseen risks, poor risk identification and management, cost overrun due to poor scoping