Why do manga translators skip chapters and when doing so do they ever intend to upload the missing ones?

If you're referring to the online scanlators, they usually do it out of their own interest. They have no obligation to do every single chapter, or problems may have arisen in trying to translate some chapters (e.g. poor quality scans, difficulty in translating, can't get hold of original scans, issues with the team as scanlating tends to be an organised group thing etc). The most you can do is contact the scanlating group and request the missing chapters, but it's really up to the scanlators themselves.