Why Don't These Welds Work?

I am creating animations by editing arm welds. I set up the welds like any normal weld:

local weld1 = Instance.new("Weld") -- left arm
weld1.Part0 = head
weld1.Part1 = falsearm1
weld1.C0 = CFrame.new()
weld1.C1 = CFrame.new(.6,.9,-1.2) * CFrame.Angles(math.rad(-88),math.rad(38),0)
weld1.Parent = head
welds[1] = weld1

And edit it like any normal weld:

firl = {.61,.62,.63,.64,.65,.66,.67,.68,.69,.7}
sec = {-1.19,-1.18,-1.17,-1.16,-1.15,-1.16,-1.17,-1.18,-1.19,-1.2}
for run = 1,10 do
    wait()
    welds[1].C1 = CFrame.new(firl[run],.9,sec[run]) * CFrame.Angles(math.rad(-70),math.rad(38),0)
end

However the arms only move some of the time. It seems to be something with the server or player because dieing or reequipping doesn't change anything, only leaving and rejoining. Even that only works part of the time. This is destroying my game that was once steadily becoming more popular. I've literally (and I mean literally) tried everything I can think of. Hopefully someone can give me some more ideas? Thanks in advance.