Can you describe this in more detail. I’d probably split the Audio around the soft section and then use the clip gain control to boost its level. But it sounds like you’ve tried that (???) and it didn’t work. What specifically isn’t working? Does the boosted section not sound louder? Can you hear a difference when you solo the Track?
If when soloed you do hear the change then that same level change should be happening when not soloed too. You can verify this by looking at meter levels. If the level is actually increasing (as I’d expect) but it still gets lost in the mix - that would suggest you have a masking problem where other Tracks are hogging up part of the frequency range.
You might also want to try out Voxcomp, which is a pretty good stock Cubase vocal compressor to even things out before just increasing the entirety of the signal. Use the output gain on the compressor to get volume back.
And perhaps give Raiser a shot (an inverse compressor = expander), in case there’s perceptual information hidden in the lower levels of the signal.
I basically compress my vocal always on the way in and then a bit more as a vocal buss in the mix. (not usually a ton of mix compression)
If I find one word/phrase getting buried, I’ll use clip gain to bring it up. Keep in mind clip gain is of course before any channel compression.
If the phrase is ‘visually’ the same level as everything and it’s still getting lost, I might then automate the track volume. (or more likely with the ‘volume’ plugin)
If you’re still fighting to get it heard, consider carving out some frequency space with the other tracks.