What is the most critical issue that can faced a market research project and lead to it fall?

selecting wrong samples, not asking the right questions, not considering the rise and fall of any political or social events that may have a wave effect during specific period of time.

Weak project managing from the start of setining the project plan till the data analysis & reporting. This weak managing can lead to a many problems may affect preject fall

I think discovering that Market share will be Zero % which means project will nt be applicable from Market point of view.

we are analyzed the particular category people to use the research. that is the main reason to issue happen.

Good Question, the answer can be taken from two sides; the agency side and the client side. 

However the issues could be linked to the research process it self: 

Starting from defining the objective, if you fail to set the research objective you will end up with results that can't be used or utilized (garbage in garbage out). 

Research instrument design, i.e. Topic guides or Questionnaire ... designing these instruments professionally and scientifically, otherwise if you fail to do so you will end up with very misleading results which putting it in action might harm your company. 

Then going to the methodology, Quant or Qual 

do you need numbers of deep understanding ? 

Then to data collection method, if you select a method that is not proper to the targeted population you might not finished the field or you may struggle a lot to finish it.

Sampling, if you target population were very rare and you don't have a list of them or a strategy to reach them, the whole project might not finished as you wish. 

actually there are too many problem on each stage of the process, however mentioning the quote: "If you fail to plan you plan to fail" would be the best on this industry.