welcome aboard, New Spey!
The difficulty you’re having is not unusual for new casters, but without seeing you cast it is impossible to effectively diagnose the problem. There could be any number of causes or combinations of causes, from dropping the rod tip on the back cast or delivery cast to improper timing to mismatched tackle and so on.
A couple of suggestions: ask around at your local shop and see if they can recommend anyone to give you a few pointers. It doesn’t necessarily need to be a formal lesson, but you need someone who is both a good Spey caster and understands Spey casting (the two don’t always go together) to watch you cast and make a few suggestions. Short of that, get your hands on a few of the great videos that are available (Rio’s “International Spey Casting” is excellent, as is the Derek Brown tape “Spey Masterclass”). If you have a camcorder take some shots of your own casts so you can compare them to the experts.
As with a single hander, in order to shoot line with a Spey you need good technique: a powerful dynamic D loop (see my article on the D loop on the “newsletter” links at
http://www.speypages.com) , a straight line path of the rod tip on the forward stroke, smooth, even acceleration as the rod comes forward, and a high, hard or abrupt stop of the rod tip at @ 11 o’clock.
That said, if you are picking up and throwing the belly of this line you are doing several key things right, and some fine tuning should take you to the next level.
One other thing—I haven’t cast your rod and line combination and am not familiar with Cortland’s new Change-A-Tips line (but I do have one of these lines on order--is this the line you're casting?). You said the belly is 57ft without the tips—how long are the tips? Reason I ask is I am trying to visualize the entire head length (belly, forward and rear tapers and any interchangeable tips). If these tips are 15ft long, for example, this means that you are trying to cast a long belly Spey line with a 13ft rod that is rated at least one line weight less than your line. This might also be part of the difficulty you’re having.