After the latest Roblox update (release 641), trails seems to have broke. They would either appear invisible on Windows PCs or broken on Mac and IOS. They work fine for the time being on Android since it didn’t receive the 641 update yet. You can notice the issue on the game Granny, a blue trail effect is meant to appear above a player or an enemy head when they get stunned by a weapon in the game, but after 641 update got released, they would not appear at all on Windows devices and would appear weirdly on IOS and Mac devices.