Custom blocks cannot be added to the theme when enabled but not the default theme, only when disabled then re-enabled.

When I installed the theme as an additional (but not default theme), all module provided and custom blocks are available for placement in theme regions. However trying to add a new block to the theme when enabled and chosen in the blocks page only adds the new block to the default (and other enabled) theme and it will not appear on mobile jquery. In order to make a new block appear I had to disable the theme, add the custom block, then enable the theme again - then it was possible to see the block in the block list and place it.