check continuity between gauge and sender - both ends of the same wire.
So if the actual sender wire that runs to the gauge when earthed doesn't raise the gauge, yet a fresh wire from the gauge terminal to earth sends gauge to max
Just for clarity is this wot u got
If so there's your prob that sender wire needs a continuity test as the mad hat man suggests
You could run a wire from sender through bulkhead to speedo plug
Then you've bypassed the wire
Try this and if gauge rises with warm up then bingo
I'd investigate why correct wire has failed later when your fresh and not so fed up
but make sure any temp wirings safe and secure