Malbury,
Welcome to the club... I had the same problem with a new K100d 2+ years ago and again with a recently acquired K200d. The problem was the same with several battery types, both rechargeable and one-shot. The simple fix in both cases was to gently form the battery contacts in the cover to give a better/tighter/positive contact to the battery poles. Do check occasionally that the contacts are clean and that they still have enough rise to make good contact. I have had to re-do the K100 contacts over the 9000+ actuations that I have on the body.
Regards
wnrr