Hello Barton!
In Sibelius I recommend adding Vienna Ensemble. You can load Synchron Player inside Vienna Ensemble.
For SYNCHRON-ized Dimension Strings, please add instruments from the Sibelius "Add or Remove Instruments" menu with “[VSL Dimension Strings I]” at the end and programs on the Manual Sound Sets page with “SYzd Dimension” at the beginning of the name. If you are unsure what to load for the "VE Strings" sound set, you can always check in the "Optimizing Sibelius Playback" manual at the beginning of the "VE Strings" chapter.
You can find more possible reasons and solutions for your problem in the Troubleshooting chapter.
https://www.vsl.co.at/en/Notation_Related
Best regards,
Andi
Vienna Symphonic Library