Long term mechanical ventilation

We have one patient with GBS on mechanical ventilation since 2 months,once we give  SBT on PS

Patient tolerates well,but still patient is not able to wean off , we plan for home ventilator.When we tried Puritan benet With flow set patient did not tolerate that even for 5 minutes.My Q is that why patient is not tolerating with flow,while tolerating with pressure support PS 8-10cmH2O?