US economy stagnant growth Why US growth is stagnant even though unemployment is significantly low . Not only US but all developed nations are facing this problem. Is limited population the cause of this , I mean lower human resources.
Not necessarily. One has to distinguish between (output - I just assume that you mean output, i. e. gdp) growth and per capita growth. 

In (very) simple terms: Growth is the result of either population growth, capital accumulation (more investment) or technological progress. 

So one reason for the low US growth could be limited population growth and looking at the data this is a possibility (Population Growth Rate US)
Another possibility might be the "unusability" of technical progress. While a lot of research is done and we discover lots of new things not everything can be used in the production progress and is therefore not reflected in gdp growth. 

One way to check this possibility is to development a new measure of technical progress - I think Daron Acemoglu and some coauthors are working on a new index. Another possibility is that this technical knowledge is just worthless. Consider the internet rush around 2000 which was at that time considered as technical progress which would increase the productivity of the whole economy... Well most of this was wrong and this technical progress did not lead to an increase in productivity - an argument often used by Robert Solow.  

So the question: "Why is a nation not growing" is pretty difficult and not easy to answer or even understand.