Can't change humanoid walkspeed after loading character

Whenever I load character, its walkspeed never changes, only works if I teleport players to specific position instead of loading their characters

You are probably loosing a reference to Humanoid.
Where are you changing the speed?

right after loading character to the deathmatch spawn (in server script I sent), something like
Character:WaitForChild("Humanoid).WalkSpeed = 60