I'm not sure why only the arrowheads appear along the path, but I have found a workaround, which is to convert your arrow into a filled path, which is a simpler type of object than a stroked line with a marker.
First, select your arrow and click Path > Stroke to Path, which will convert your arrow into a group of two objects (the line and the arrowhead), which is 90% of what you need to do. However, sometimes this conversion is a little messed up (see image below, where the line is extending too far to the right, and poking out the front of the arrowhead).
To fix this, use the node tool (F2) and select the arrow shaft, and you should see the nodes appear for editing. Select the nodes on the right side of the line where it's poking out on the right side of the arrowhead, and use your left arrow key (←) to move the nodes back into the body of the arrowhead.
Now we want to merge these two objects into one composite object. Select the group of 2 objects and ungroup them (Object > Ungroup). Then perform a Union operation (Path > Union) to merge the two objects together. You should be able to use the node tool (F2) to confirm that this is now a single object:
Now you should be able to use the
Path Effect Editor to put this object along the path. Then use the fill and stroke dialog to style the arrows along the path.