4 Most Common Reasons you might see a spinning wheel or non-functioning PayPal popup:

-- Using Preview Link to test instead of live link...   Solution:  Always test using the live link url.

-- Config in Test mode, and PPv2 Integration is Live...   Solution:   they should both be the same.

-- Required Fields may not be filled in..   Solution:   double check required fields are filled in.   Pro Tip:  Use this in conjunction with SCROLL TO ERROR add-on so if there a required field not filled in, the page will automatically scroll to the error.

-- Domain Name in use should also be added to the Domains area in CFProTools so the domain is whitelisted for usage.