Timing is extremely important when asking users to take action on your website. Placing your call-to-action too high on the page can deter users from taking that action, simply because you haven’t presented them with enough information to be comfortable in proceeding. Generally the more expensive or complex the product or proposition, the more information you need to provide before people are ready to take action. The Fogg Behaviour Model explains this well - users will take action if you present the trigger (call-to-action) when their motivation is at their highest and it’s easy to take that action. Figuring out when this is can be tricky, but research methods like usability testing can really help. Running testing sessions with a number of representative participants over a prototype or even the current site can identify where users are getting stuck, and when they’re ready to take action.