I have configure multiple payment gateways.
When user select paypal some time it is redirect user on cart page instead of paypal.
I have marked that user is on http://domain.com/store/paypal/express/start/
url and then redirected back on cart page.
Please check the console. Also check the log files. If they describe any error then try to solve that error. And when you get redirected on the cart page, is there any error message shown on page?
I am having the same issue. After a customer is directed to PayPal, they login and pay, the redirect hangs up here: