Why doesn't my quick bread rise properly when substituting splenda for sugar?
If it is a quick bread then it should be chemically leavened with baking powder or soda.
The presence, or absence, of sugar should not play a role at all in the working of baking powder.

Where sugar may play a role, however, is in creaming the fat. If this recipe calls for solid fat such as butter or shortening then it will often also call for the sugar to be creamed with the fat. This step is very important as it will create the millions of little bubbles that will define the texture of the finished product.

You can try adding more soda which will affect the flavor. You can try beating the butter more on it's own. I have read that some people folded beaten egg whites into the batter.
I can't recommend one approach over the other. 

My personal opinion is that if the recipe calls for creaming butter and sugar then it is not a good candidate to use splenda.