Problem with network drivers

We are currently developing an application that communicates with several
retro computers. This means that we need to the ethernet adapters to
transmit/receive no tcp/ip or ftp packages. Our application is up and
running for all PC configurations using the library. But our
packages appear to thrown away by drivers devn-speedo, dev-tulip and
devn-tigon3. What is different with the devn-rtl driver? All adapters work
OK with the Windows version of the application.