Python Selenium未单击按钮

Why would selenium not find the element using xpath?

<input class="btn btn-success" name="submit" id="loginButton" accesskey="l" value="Login" tabindex="6" type="submit">

使用:

driver.find_element_by_xpath("//input[@class='btn btn-success']").click()

我得到:

selenium.common.exceptions.NoSuchElementException: Message: Unable to locate element: //input[@class='btn btn-success']

也尝试过CSS选择器