This could be caused by a number of different things. Would need to see an example of the experience or some log files or something more. Are you working with a specific plugin, or are you working with the basic PayPal that is included with WooCommerce? I would recommend you check our PayPal for WooCommerce plugin. It has a lot more features and functionality than the basic PayPal with Woo, and you shouldn't have this problem with it.
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