Cisco mmf to smf help

Alright, I've got a headache over something that should be so simple, but is getting complicated by others.

I'm a network infrastructure technician. I've just completed a fibre link to two job trailers(1x12f sm to each trailer). Single mode fibre was used. The smf runs from the trailers to a junction that has existing multi mode fibre that links back to the head end.

Now, my first thought was a mm-sm media converter in the middle, but the Cisco switches at each end don't seem to like this. I get activity on the media converter, but not the switches. So we stuck another Cisco switch in the JB and have the connection working, but this is only a temp solution(that's a really expensive media converter, admins don't like cascading switches). I've got a couple more media converters for fibre to Ethernet standing by to place on each end, but I'd like to hear if there are any other options. Diagram below.

Head end-mm fibre-junction-sm fibre-trailers.