Layer 2 of support not feeding

[Monkeyboy]

I've been trying to work out why my first attempts at using ABS & HIPS have been making birds nests and have zeroed in on some really weird behaviour. In the course of my investigation, I've eliminated the raft from my enquiries and am now printing directly onto the bed which starts with a large support area.

As per the video below, layer 1 of the HIPS support, right nozzle in red prints fine. As does the first layer of the four little legs of my test piece in black ABS from the left nozzle.

Moving up to layer 2, and.... nothing. It's printing air. In other words, the printer's doing everything except feeding the filament. Please see my short video below:

https://www.youtube.com/watch?v=yJIsdg_VQMo

Any ideas?!