What's The Real Problem For Builders?

In my discussions with builders,  the most common refrain I hear is, if I could get more labor, I could sell more homes.  I have also wondered about how much the financial problems, from student debt or affordability and disinterest in home ownership of younger, first-time buyers is slowing growth.

I don't hear builders tell me that first-time buyers are holding them back today, they say it is labor.  When we were at the peak of  the market, before the crash, the availability of materials was sometimes a problem.

I recently read an article in the Wall Street Journal that the availability of affordable, desirable land is holding back some builders.  

I assume this is a geographic issue.  Finding land in Toledo, Ohio is probably a lot easier than in Denver, Colorado.

So my question is which of these three issues is having the most effect on builders?  Is it labor, first-time buyers or the availability of land? Or is it something else that I haven't even thought about?

What are your thoughts?