Sorry, I confused POC with Completed Contract...
In case of POC job, WIP cost is always zero, because of POC definition.
To post WIP sales, "Billable" (or "Both Budget and "Billable") job journal line with line amount rather than zero should be posted.

If the posting is done without problem, job task line should be like below.
Important point is Actual (Total Price) is not zero. You can add this column using page design feature.

And also check %Completed is not zero.

Calculate WIP.

Then Sales WIP will be calculated.

I hope this would help you.
Thanks,
S.Kawamura