Forms Authentication Cookie Not Created

We have a new instance of NOP 3.9 setup however when trying to login the NOPCOMMERCE.AUTH cookie is never created and the login page redirects back to the homepage. Anyone know what might be causing this issue? IIS Configuration issue perhaps?

barnold wrote:

Please check question "Q. Users are logged out frequently"  on
https://www.nopcommerce.com/boards/t/27211/frequently-asked-questions.aspx

SuperNopCommerce wrote:

I have set a machine key however setting a machine key has no effect on this issue. The only cookie created during the login process is the Nop.customer cookie. We have the exact same code and web.config running on another server and it works perfectly fine.

I just Uninstalled IIS and Reinstalled it now the cookie is being set properly. I wish I knew what the problem was though without having to reinstall IIS.