I am fairly new at Intarsia but here are a couple of tips I have found work plus a link to a video.
At a recent seminar, Judy Gale Roberts recommended that you use a magnifying glass to cut all adjoining pieces. She then cuts to the line exactly. I have tried but every magnifier I tried duplicates the line instead of enlarging due the astigmatism in one eye. I therefore go real slow. Also, I find it easier to cut red lines versus black.
Also, it is very important the your blade is exactly 90 degrees and when you cut corners that you do not let the blade bend. Recently I got a flat master (similar to a Sandflee) and found that a lot of boards that I thought where flat had a slight warp and a couple of passes on the flat master made it perfect and made the match even better.
There is a video series listed below that is fairly lengthy but part 5 talks about repairing gaps. It works quite well.
https://youtu.be/npTeGSGGxjYIn all it takes lots of practice and patience.
Don
If all else fails a little glue and saw dust in the joint works too.