Autosnap - what am I doing wrong ?

So I've got a track that I'm mixing for a friend - vocal, bass and acoustic - and the timing is really really sloppy.

I can't get the parts re-recorded, so I'll just tidy up the timing with Autosnap - makes sense and having watched the video, should do the job.

I set the tempo to the rough level, and then starting from the left, find each bar transient across each stem, and drag it to the correct bar marker.

After more than an hour this is all going well, and whilst it sounds a bit rough - I assume due to the 'realtime' algorithm - I'm confident it will render down to a decent sound. But then I hit a problem:

For some reason, there's an implied transient at the end of the clip, or so it seems - everything to the right of my work has been getting more and more compressed, such that as I near the end the remaining audio is extremely fast :(

I'm at the point now where I can't move the next transient far enough to reach a bar, and I think I've maybe wasted all of my time because I can't find a way to fix this - there doesn't seem to be a way to 'extend' the audio clip total length so that there is more space to move transients.

I'm sure there's something obvious I missed here, because it seems like a perfectly normal requirement.

Any ideas ?

Thanks.