通过后台返回的orderString只能调起一次支付宝app，更换orderString则无法再次调起

在做支付宝对接时，后台返回的orderString第一次我可以调起andriod的支付宝App，只要不支付可以一直调起，但是如果我重新请求，更换新的orderString，则无法再次调起，这是为何。

附上第二次的orderString(app_id)：
alipay_sdk=alipay-sdk-java-3.3.4.ALL&app_id=2017080208007475&biz_content=%7B%22body%22%3A%22%E7%94%B5%E5%AD%90%E4%BA%A7%E5%93%81%22%2C%22out_trade_no%22%3A%22train_pay_156094043500%22%2C%22product_code%22%3A%22QUICK_MSECURITY_PAY%22%2C%22subject%22%3A%22%E5%9C%A8%E7%BA%BF%E8%AF%BE%E7%A8%8B3%22%2C%22total_amount%22%3A%220%22%7D&charset=UTF-8&format=json&method=alipay.trade.app.pay&notify_url=http%3A%2F%2Fxm.appgy.pw%3A38080%2Fapi%2Ftraining%2Fnotify_url.json&return_url=http%3A%2F%2Fwww.xxx.com%2Falipay%2Freturn_url.do&sign=KJ4NyofKzL10Ljh7PX0W6N8neLlA2VzvHJUiZv%2BLXee%2Be447iwcyYu13lVAMEzaHtS6Lh%2FJQDW%2FEj0Htrj%2BVC58ikRVUXI2A1SRASlEsMkyVH4uDaUWmWAd3IGiHK9uwa2RvISTgikWtDGbkQCGHgsvz3m%2FVuK3ZE8FxkDpZzImoc%2Bis9SSY2AiRX1R58e3F4oVUUbi7k7SDr%2BD2%2FD8byJv8DNmquHfgxKeMQHy45h2vdLKxJv4MndHHBQlJSE5gyfAx8UmG9vYouxEclxlJkjjAVZLJJsCQ%2Fe3wZhTD4lmndg%2FsvpeU2SPivZAtj7HaNfUXX5MHbcMjP%2B1xxt8AfA%3D%3D&sign_type=RSA2&timestamp=2019-06-20+13%3A23%3A17&version=1.0