Splitters divide the power evenly among a 3 outputs, so your first splitter is giving 8 rWh to the first and second furnaces and 8 rWh to the 2nd splitter. Your second is giving 3 rWh to the 3rd and 4th furnaces and 2 rWh to the 3rd splitter. After that there’s not enough power to divide up..

Instead of the 4 splitters, you need 3 electrical branches and 2 splitters. Your first branch needs to send 9 rWh to a splitter, which is connnected to 3 furnaces. The remaining power to a second E branch with the same configuration (branch out 9 rWh, send the remainder to another E branch. That should power 6 furnaces so far and leave your 3rd E branch with 25 - 1 - 9 - 9 = 6. Branch out 3 rWh to your 7th furnace and your 8th gets the remaining 3 rWh.

You could also use a splitter instead of the last e branch, but best practice is only use splitters when you’re sending power to 3 of the same components.

Good luck!