Some orders won't post to shipwire

On the shipwire module for Magento I'm getting the following error when I click the submit to shipwire

Unable to read response, or response is empty
Trace:
#0 /lib/Varien/Http/Client.php(56): Zend_Http_Client->request(NULL)
#1 /app/code/community/Meanbee/Shipwire/Model/Api/Ordersubmit.php(35): Varien_Http_Client->request()
#2 /app/code/community/Meanbee/Shipwire/controllers/FulfillController.php(26): Meanbee_Shipwire_Model_Api_Ordersubmit->submitRequest()
#3 /app/code/core/Mage/Core/Controller/Varien/Action.php(367): Meanbee_Shipwire_FulfillController->submitAction()
#4 /app/code/core/Mage/Core/Controller/Varien/Router/Standard.php(247): Mage_Core_Controller_Varien_Action->dispatch('submit')
#5 /app/code/core/Mage/Core/Controller/Varien/Front.php(158): Mage_Core_Controller_Varien_Router_Standard->match(Object(Mage_Core_Controller_Request_Http))
#6 /app/Mage.php(457): Mage_Core_Controller_Varien_Front->dispatch()
#7 /index.php(52): Mage::run()
#8 {main}

It only happens on some orders and not all and I cannot find out what is the variable that is causing the issue. Is it magento, shipwire or the module?

The interesting thing is if I hit back and then click the same button again to submit the order it has always worked the second time but then says that it is a duplicate order and states that it is ignoring it and marks the order in magento as complete.

Any help would be great.