The way the levels work in AvP Classic is a little unique. Every level has flags that indicate which predator weapons (aside from the wristblade) are permitted and prohibited. So for instance, the first two predator missions have the pistol and disc prohibited, while the last mission in the campaign, Battle, prohibits the shoulder cannon, medicomp, pistol, and disc. You can cheat those weapons in, but the maps themselves are coded to have those weapons excluded, which is why you're getting that result.
Since I have the editor, I easily enabled those flags, granting you the ability to use all predator weapons in that map. You should be able to use all the weapons even without the GIVEALLWEAPONS cheat.
Link:
http://www.mediafire.com/file/xpxweqb80v6yett/Battle_All_Weps.zipPut the Battle.rif file in your Steam\steamapps\common\Aliens versus Predator Classic\avp_rifs folder and use it instead of the original Battle.rif file.
Edit: Hit me up on
Steam too so we can maybe get a game in.