I have two files, one with a topic and one with definitions:
 TOPIC
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE topic PUBLIC "-//OASIS//DTD DITA Topic//EN" "Topic.dtd">
<topic id="CREF_TOPIC">
    <title>Topic with crossref</title>
        <body>
        <p>The best thing since sliced bread is <ph conref="cfs.dita#cfs/mpm"/>.</p>
    </body>
</topic>

DEFINITIONS
<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE topic PUBLIC "-//OASIS//DTD DITA Composite//EN" "Topic.dtd">
<topic id="cfs">
    <title>Cross References</title>
    <body>
        <p>
            <ph id="mpm">My Payment Method</ph>
        </p>
        <p>
            <ph id="stx">SuperTransactions</ph>
        </p>
    </body>
</topic>

If I set this up & view in Oxygen 10.2, the first tile (TOPIC) displays the cref values just fine.

When I run any of the default transforms in Oxygen (to PDF, HTML, etc.), the cref value is blank. I'm not getting an error in the transform - says it transformed fine. But cref isn't working.

Clues on how to debug this? The cref is a major component of my documentation plans - I want to have pre-defined values that I can use in documentation so I don't need to update product names, release dates, and so on, inside the final documents.