This happens because the new block created changes the index of the blocking. That is, if the range of blocks is 2x4x6 for example, Hexa shows the range of 2x4x6 blocks. This index allows you to hide some of the blocks and work on specific regions of blocks, which can be useful for a very complex geometry and blocking. When you create a block outside this original range, i.e. extend from one side of the geometry outwards to make the range 2x4x7, Hexa keeps the original range visible and this is why you cannot see the new block. To get around this problem, right click on Blocking, click on Index Control and click the reset button. This should then show the whole range. Please also note that the same thing happens if you make an o-grid inside another o-grid. The block range will then be 2x4x6 – x2 (in the o-grid direction), but will only show one o-grid direction block and therefore the second o-grid will appear to disappear. Again, Reset will fix this problem.