EDI Base Adapter - 997 Ack question

We have a scenario that goes like this: BizTalk 2006 generates 4 single EDI

850 files. They get sent to the vendor. The vendor replies back with only
one 
997 ACK, that looks like the following:

ISA~00~          ~00~          ~14~VENDOR           ~ZZ~BIZTALKSENDER     
  
 ~070404~0901~U~00401~000000482~0~P~<
GS~FA~VENDOR~BIZTALKSENDER~20070404~0901~482~X~004010
ST~997~0001
AK1~PO~188813
AK9~A~1~1~1
SE~4~0001
ST~997~0002
AK1~PO~188811
AK9~A~1~1~1
SE~4~0002
ST~997~0003
AK1~PO~188816
AK9~A~1~1~1
SE~4~0003
ST~997~0004
AK1~PO~188821
AK9~A~1~1~1
SE~4~0004
GE~4~482
IEA~1~000000482

For some reason, only the first do***ent seems to get acknowledged (# 
188813). I verified this in the EDI Re****ts function of the HAT tool.  All

the other ST/SE segments seem to be ignored, so those do***ents claim to
be 
still waiting for an ACK. Is this by design? I would appreciate any 
suggestion.

Thanks