The most common issue with netplay is matches desynchronizing (desyncing). The problem almost always stems from players using different versions of the Melee ISO. Always verify your file's MD5 hash matches the official standard to avoid desyncs .
Modern netplay clients, custom training codes, and major mods are built specifically to alter the v1.02 codebase. How the 1.02 ISO is Used Today super smash bros melee 102 iso