I put together a Plunker example here using the Angular UI and basically copying the code from the question. I then took this example and added an ng-repeat to demonstrate one of the most common issues I have seen: scope issues:
<div ng-repeat="x in collections">
<input type="text" class="training-comment" placeholder="Enter comment"
ng-model="content" data-ui-keypress="{13:'keypressCallback($event)'}" />
<br /><br />
</div>
You can find this updated plunker example here. Basically whenever you use an ng-repeat or any other directive that creates a new scope your model exists in that scope - not the parent or root scope. This means that your code might be working, but it is printing out the value from the wrong scope! For more information on scopes see the angular documentation here.
To use the plunker demo, type into the first input and press the enter key, the model will be updated. But, if you type into either of the other two boxes, though, the model will either not be updated or it will be undefined (if you have never typed into the first box).
Even if this isn't the exact issue, hopefully it will still help. Best of luck!