Why have I not grown in the past 6 months?

That really depends on how old you are. If you are in your 20's, it's unlikely that you will grow at all from now on. Growth stops around mid 20's to early 30's.

If you are currently experiencing puberty, it is possibly you have already hit or not yet hit your growth spurt, in which case the lack of growth, while unusual, is not worrisome just yet. If prepubescent, this is also rather uncommon and it may be you are mistaken and you just feel like you haven't grown. If you really haven't grown and you feel like you should, ask a doctor. On your own you can drink milk, eat nutritiously, and sleep well as these all aid in growth.