When LabVIEW finishes executing a VI it will destroy open references when the VI stops running. The exceptions to this rule are VISA and IVI references. VISA/IVI sessions have a preference setting that closes their references.
To get around this problem, you have 2 options.
1) Upgrade to TestStand 2.0, or if you are currently using this version, check the box titled Reserve VIs for Execution. This can be located by selecting Configure>>Adapters and selecting LabVIEW Standard Prototype Adapter.
2) You must start a daemon VI running that will continue to run throughout the duration of your sequence. This VI will be responsible for creating references and returning them to the steps that requested them. If you visit ni.com and search for "+teststand +references" yo u will find an example titled "Creating Permanent LabVIEW References for TestStand" that does exactly this.