In my last job I used to get this, I put it down to tension in the ribbon not being equal and mainly happened at the edges – for most jobs it would be fine but generally happened on jobs with repeats, Also the first few that printed it did not happen on. I used a workaround that did used to work this was to place a line of small ovals in front each print just to even up the tension in the ribbon. It sounds like a lot of hassle and waste but in the long run probably saved us pounds in stickers that we didnt have to throw away.
This was an occasional problem, I could print one job fine but go and repeat the same job next day and I would have this problem – so perhaps temp / humidity is an issue. Note that I came up with this work around because all other factors failed, including changing speed, profiles and this was also using third party inks + supplier not able to help