R.R.
I'm guessing too much fat, shortening or butter, which will cause the cookies to flatten out, rather than rise. And thinking the humidity may have played a role as well, though I've never really noticed it with cookies, primarily when making candy or melting chocolate.
I did find some advice on making/baking cookies:
http://allrecipes.com/HowTo/perfect-cookies/detail.aspx
Hope the cookies taste good anyway : )