Graph X axis (multi-level category labels) not showing all the text

I have created a graph with multi-level category labels by weeks, months and years, however some months is not appearing on the graph x-axis. I have adjusted all the font size but still with missing months (ie Jul, Aug & Sep appears in the graph but Oct, Nov & Dec are missing). Greatly appreciate receiving assistance on how to fix this.

Thanks Andy for your reply. I set to 1 the Interval between labels but still not working. I agree this is excel bug. It works fine if I have to hide (or limit the axis the reference) some of the column data reference.