I am trying to book a ticket JFK-TLV on DL (metal, next March) and return TLV-EWR on CO .

I need to fly CO on the return is b/c I am traveling with a companion on the TLV-EWR segment (in coach). However, on the outbound, DL can confirm an upgrade at time of ticketing (from Y/B/M) class, where CO can't confirm anything. So I'm trying to grab this upgrade and burn 25K DL miles.

I was able to get a combinable "H" fare on DL outbound for a total of ~$1200 RT, but when I asked to be booked in "M" class on the outbound, they couldn't find ANY combinable "M" fare. The lowest combinable Y/B/M fare was a "B" fare for a total of $1950. I was hoping to pay something in between.

Is it common for DL to have an "H" combinable fare but no "M" combinable fare? Any suggestions anyone? Thanks