How do you make two separated fuel tanks feed into each other?

I made an ICPS for the SLS and the Delta IV Heavy rocket, but the upper tank's fuel isn't being used, so the rocket doesn't have enough Delta-V to make LDO. I have struts connecting one fuel tank to the other with fuel transfer on, as well as an actual fuel tank connecting them in the middle. In theory, it should be working fine, but the hydrogen tank's fuel is never used. Any ideas? By the way, it was working before on the previous ICPS in the SLS, it has only been failing since I updated the stage to look more accurate to the real thing. It doesn't work on either rocket, I've checked. Please help.

By the way, the decoupler between the two also has the 'fuel line' option on, but it isn't working :/