NetworkTransport.Send not flushing send queue? (LLAPI)

When I do NetworkTransport.Send(...) I receive the message on the receiver fine but the GetCurrentOutgoingMessageAmount() continually increases on the sender...

until finally I get the message events are full error (something like that) even though the messages ARE being sent... Network.Send fails after hitting this limit.

Note I'm using a reliable channel for Sending.... Do I have to manually return Acks or something... this has got me really frustrated.