AAC Encoder returning MF_E_TRANSFORM_NEED_MORE_INPUT despite feeding it plenty of input

Using Media Foundation to generate MP4 audio and video file.  Don't want to use the file sink approach - we are generating the container ourselves and just need the transform to compress the audio data.

The video H.264 part is working fine using the Media Foundation H.264 encoder.

However, using similar code to do the AAC audio does not work.

This line:

FAudioMFTransform->ProcessOutput(0, 1, &FAudioMftOutputDataBuffer, &status);

returns MF_E_TRANSFORM_NEED_MORE_INPUT, regardless of how many times ProcessInput is called, and despite all transform configuration, media type selection etc. succeeding.  (ProcessInput also succeeds).

How can I track down why this is happening?