After importing an IDX file, the traces are not imported.
Solution
If you open the IDX file into an editor, search for the line that contains <pdm:LayerPurpose>. If the line has a value of PowerOrGround or OtherSignal it indicates that traces do exist. If the value is LandsOnly, this is for pads, if the value is SolderMask this is for Masks.
Has Traces:
----------------
<pdm:LayerPurpose>PowerOrGround</pdm:LayerPurpose>
<pdm:LayerPurpose>OtherSignal</pdm:LayerPurpose>
No Traces;
---------------
<pdm:LayerPurpose>LandsOnly</pdm:LayerPurpose>
<pdm:LayerPurpose>SolderMask</pdm:LayerPurpose>
Hardware/Software Configuration
Platform: all
OS: n/a
OS Version: n/a
Product: NX
Application: PRINTED_CIRCUIT
Version: V1867
Function: PCB_EXCHANGE
Ref: 002-8516245