I booked an itinerary on expedia with 4 Delta segments and two KLM segments, however, all the flights are KL coded.
The KLM site would not recognise my status and hence would not let me select premium seats. On the DL site, I could not even make a seat selection. I figured maybe it will take some time as it was an expedia issued KL coded ticket. After waiting for about six hours I called DM line. The agent said "You are not supposed to be able to see seating charts. Even I cant see them"

I asked her how I was supposed to select seats? "They will be assigned at check in as they are all KLM flights" I pointed out to her that 4 of the flights are on DL metal. But she insisted they were KLM flights. She didn;t seem to know the difference between DL coded and DL operated.

Another call to the DM line and intervention of a supervisor got me seats assigned on all six segments.

The only seats in EC left were middle seats. Doesn't DL save some seats for gate check assignment? According to the supervisor, they release some seats before gate check-in if they are needed.

What routing are you flying and when?

Depending on the route, (e.g. to/from LHR), I've seen some EC bulkheads reserved for airport issue. I know on these flights 15 AB/FG are usually airport assignment.

DTW-AMS (333) on Dec 2nd and AMS-DTW (767) on Dec 7. If EC bulkhead seats are reserved for airport issue, how come all the bulkhead seats, except the middle seats, have already been assigned? A supervisor on DM line could not answer that question.

I've only seen the EC seats I mentioned (on the 764) reserved...every booking I've looked at to/from LHR has them blocked. On the A330, they are usually all available. I have no idea why DL is not consistent about this.

Anyway, have you tried to look your itinerary up on klm.com or airfrance.us since it's on KL ticket stock?

This whole things sounds very odd...

It does sound odd. In the past, I have always been told that bulkhead seats are released at the airport. I don't ask for bulkhead seats, I prefer a window seat but can take an aisle seat also. I used a CPAP which I place on the floor in front of me. If I am in a middle seat, my neighbor could inadvertently step on the device or get tangled in the hose.

DL offers up most of the EC bulkheads in advance.

On the A333, I believe they hold row 29 for airport control.

So you give up EC if you want or need bulkhead?
On KLM site, I still can;t access premium seating. The KLM site shows my SKymiles number under Flying Blue number. Does KLM site recognise staus from the Skymiles number? Why does it show Skymiles number as if it were Flying Blue number?

I don't know if this has anything to do with your case, but I had kind-of-a-similar experience with a DL coded AF operated flight.

I could see the seating map on the DL site and it would let me select seats but refused to save it. When asking DL they said they couldn't help me with an AF flight and I'd have to wait 'till the AF check-in. However, when I turned up at the airport we had the exact premium seats (744 upper deck) I selected at the DL site reserved for us, so apparently the selection got through even though the system did not confirm it. Could this be a similar case if you can see the seating chart but not reserve a seat?

Same thing on the way back, except they moved us from middle and window to aisle and window with a middle blocked, which I did not protest too much.

I've got an orbitz booked DL itinerary coming up at the end of January. Once I hit the 90 day time frame I called the DL diamond line and got assigned KL EC (or whatever they call it) seating for the two transatlantic flights. A few weeks later the return schedule changed and I called again and got rebooked from KL on the return to DL AMS-DTW. Again straight into EC.
At that point the entire itinerary disappeared on DL although it was still there on the KL www site with the KL PNR that I'd gotten from an agent.
A recent email to talk to us resurfaced the itin on delta.com, seats still in tact but with a new ticket number. Seems they reissued the ticket but didn't change the PNR when the fixed the schedule change. Needless to say, I'm watching the trip closely.