Can't bounce through bus/aux… sometimes!

I've been using the following process for weeks, but today it's suddenly stopped working and I really can't figure out why.

The bounce process:

* Route all (audio) tracks and buses to bus #40 (which has a few plugins and balances the level)

* Route bus #40 to #41, which is a stereo audio track in the Arrange window

* Hit 'input' and record.

Today, the exact same process works sometimes, and other times not... I'm finishing an LP and really need a solution.

FWIW, bus #40 receives signal, but I can't then send that to a track for recording.

The following things I have checked:

* Any mute or solo buttons hit in Arrange or Mixer window? None.

* Any automation? None.

* Alternate buses (#43, #23 etc)? Same problem, no signal passing to the desginated bounce track.

I've restarted LP, my mac and run through the gamut of I/O buffer size settings (which usually fixes random behaviour). No joy.

And, yes, for MIDI I can bounce in place. But I've mostly an audio project.

(LP 9.1.8, Mac Mini, OSX 10.7.5, 16GB RAM, 2.3Ghz CPU, Tascam US-1800, recording to external Iomega HD 7200rpm).