I had this problem last week with 983 (DUS - IAD - ORD).

I booked to go DUS - IND. Originally I was booked 983 to IAD and then nonstop to IND.

Then I called back to add IAD - ORD - IND (589 and 500 miles respectively versus 476 IAD - IND nonstop).

Problem was the first IAD - ORD left 45 minutes after I landed (unable to book). Next flight was 983. Booked me on 983 and unable to be ON the upgrade list.

When I got to IAD, went standby (FC) on empty A320 (FC full of employees, some working some not) from IAD - ORD to avoid being on 983 entire time.

Weirder yet, I have only received MP credit from DUS - ORD nonstop on 983 (which I didn't fly obviously...)

One of the MAIN reasons I love the 1P/1K change any time without having to fly stand by rule...