Hi I need help, please

I'm using the tcpip-stack (tcpip.inc) from Erik Andersson and I have a problem:

I can establish a connection and everything, but if I send a packet including only an ACK for any data and then I try to send a packet including data, the server/client doesn't accept it - Why

I have a programm to monitor every packet on my connection and every packet seems to be OK: seq.nr, ack.nr, IPs, Ports are all right maybe its the window, its only 65bytes?

my hardware:
SBC - 80552
Crystal-Chip CS8900A - 8bit mode

my email: harald.huber@a1.net