I have done some thorough testing of the battery holders on the original v0.3 boards and the new v0.4 boards and found that it’s the texture of the bottom of the battery that makes the difference. It appears that one brand of battery might have some problems making good contact with the board, but this isn’t limited to the Mayfly. I tested 3 different brands of batteries and I found that one of them is almost smooth of the bottom, whereas the other two have a bumpy texture in the metal. See the attached photo for a comparison between two of them.
What I found was that the smooth-bottom battery sometimes has trouble making good contact when inserted into the Mayfly battery socket. I inserted the bumpy-bottom battery into over 100 Mayfly boards and every one of them worked perfectly. I also had trouble with the smooth battery in a few other devices that were not made by us, so I think the problem is caused by the battery and not specifically the Mayfly board. In every case where the battery wouldn’t make contact, putting a small bit of solder on either the battery or on the board under the holder immediately fixed the connection issue.