Well, I'm no expert, but have you got any round based scripts (or any scripts at all) on your map? If so, try turning them off and playing a round? I'm not really sure.
Do other weapons also stop dealing damage in the hands of the infected? Or is it just the sniper? Does the sniper still work for survivors? Does the sniper have any scripts? How is it spawned? (Loadout, Weapon Pad, or "dropped"?) - *Note: I am by no means an expert either, but I figured I could try to help.
no scripts on sniper, but i did recently add a despawn at start then spawn it into the match to see if that fixes things, havent tested it yet. only infected use the sniper. I did once pick up a rocket laucher as infected and shoot at the survivors, one should have died but he didnt, so its not just an issue with sniper. I dont think its a weapon pickup issue either as ive tried starting infected with a sniper and the same thing happens. infected all start with shotguns and the shotguns dont have issues but its hard to tell with a shotgun as you dont always get a chance to use it. but i havent experienced any issues with it.
It may be a gametype setting that Infected can't deal damage with guns. Are you using standard Infection?
Are you using a custom game-type? Check the damage tables, see if the infected deal reduced weapon damage or if the survivors have increased damage resistance. If you aren't using a custom game-type, then I'd recommend that you try it. If you have issues with making your own custom game settings I would offer to help out personally, but I'm not sure if I can. I'd recommend grabbing a friend and testing all available weapons in custom games. That way you can see if all weapons are affected at the same time. This is the best way to diagnose a problem with damage values, and I feel that's where your problem is. - Similar to how the old "shieldless" Breakout game mode actually had "mini" player shields (Observed this myself by taking damage, then seeing my health regen at 2 different speeds), though the shields were hidden(Displays the value via the health bar instead, making it look like you have no shields) and default damage values were modified to suit "Breakout". For example, 100% shields/health in Breakout may have been equal to 50% shields/health in regular Slayer (Not personally tested). This leads me to believe the same could be said for Infection; as the infected are only supposed to use swords, and preemptive measures were probably taken by lowering the default weapon damage in case a bug allowed the infected to use weaponry in online matchmaking. This would also explain why melee damage might be increased to 200% or 300%(Haven't looked at infection numbers in a while), as there's no need for a melee buff unless there's a base damage nerf...assuming you still want the energy sword to 1-hit.
some rounds it works perfect and then some rounds it deals no damage at all, I had it even not work at the start of a round then suddenly start working half way through the round, not sure how I would debug that, seems too random. it must be the labels im using when applying them to players but those scripts are vital to how the map works.
Hm...that is extremely bizarre... You're going to have to test the map without those labels. If they're the problem, then you'll probably have to find a better way to implement the desired function if you want those guns to work. Certain scripts interact/behave weirdly, unless those issues have been fixed. However, you said the damage was wonky even when spawned as loadout weapons...so you've got yourself a very bizarre situation.
think i might have found a fix. i was removing labels at the beginning of a round (to reset each round) instead of during the previous round which kind of fits the pattern of my problem. Ive set the label to remove after people die or when they get to the end of the map so that it doesnt have to do this on a new round. when a player gets applied a label he gets a unicorn head. this unicorn head cant be removed during a round, the label can still be removed though so i thought everything should be ok. the unicorn head must have been causing issues. will test later.