HELP! Paths seem to change in my illustration when I save as a SVG

First time trying to use illustrator as needed to convert some illustrations into vectors.

I have opened up my first file and seemingly on the Ai interface successfully converted to a vector and made my bckground transparrent, but when I then save as an SVG and open, several of the edges between colours have a transparent gap that isn't visible in Ai so I'm not sure how I can get rid of them!? If anyone is able to help (using as simple language as possible for a newbie!) I'd be eternally grateful!

Pics attached are zoomed in - the dodgy one has been placed on a white background, its still transparent.