I think I had a similar issue. Ableton listens for all the sended midi commands and only maps one of them. If you have more than one modline in the softstep editor per key, and each modline is sending a different midi command, than mapping Ableton could be tricky.
You could try the following workaround (it worked for me):
1 - Go to the softstep editor and turn off all modlines, except the one with the midi note you want to map
2 - go to Ableton, and in midi learn mode, press the sofstep button. It should map correctly
3 - go back to the sofstep editor again and turn on the modlines.
hope it works