Try at the gate that JL is using in LAX once JL has their staff there.
This year I was flying:
PHL->ORD (US)
ORD ->NRT (JL)
NRT->FUK (JL) with NRT->FUK on a separate ticket. PHL, ORD check-in and ORD FL couldn't check me in or issue the NRT->FUK boarding pass. JL's website was unable to check me in for NRT-FUK.
The JL staff at the gate they were using in ORD for the ORD-NRT could.