Melodyne clip becomes empty of audio when rendered

I render a melodyne clip and the audio disappears. Strangely, it is only happening to some of the melodyne clips; other melodyne clips in the same track are rendering fine, which doesn’t make sense. This is happening in several projects that I am mixing at the moment.
 
I might have maybe 10 or so melodyne vocal clips in the one track, each of them created using the create region fx tab. They are all healthy looking clips with visible waveforms. I render one clip at a time using ‘bounce to clips’. Some clips render fine by this method, but others have the waveform disappear completely after rendering. There is no apparent difference between the clips that render perfectly and those that don’t – they were all created in the same way.
 
A couple of times I have had success by closing the project and opening it again and then rendering an offending clip and it has worked. But there are some clips that just won’t render properly no matter what.
 
It’s not a muted clip, and it looks and plays fine before rendering.
 
The track is not in mute mode. As the clips which fail to render share the same track as other clips which render successfully, it has nothing to do with the status of the track.
 
I have tried closing take lanes before rendering as sometimes clips act strangely when in open take lanes.
 
I have tried rendering by using the ‘render region fx’ command instead of the ‘bounce to clip’ mode and I get the same result.
 
I tried it in Sonar X3 and the same result.
 
I am using a full version of melodyne, version 4.1, I think.
 
Anyone with any ideas?