From the sounds of it, you have not done a full troubleshooting to fix.
Pls proceed to flowchart 9 in the chart that also stated “no leaks were detected in the System (Ventilator) Checkout” for this issue as you described.
It will follow actions like - Test the Inspiratory and Expiratory Check Valves (Section 7.5.1), then step by step before probably replace VIB board, and so on for this issue..