Hello Mike

I don't own a DE, but this sounds like the CV from the interface is fed
via a switching contact in the VCO-F socket to the VCO. If this
switching contact is worn out, you won't have any longer a control from
the MIDI-interface.

If you dare to open the DE, then you might repari the socket like
described in http://www.doepfer.de/faq/a100_faq.htm#contact%20problems

Florian