Hi Steve, and welcome back to JA!
The drawing is only based on how many bonds you own. Not on when you bought them or if they were purchased in blocks. The odds of winning a prize on each bond, each month, is currently 24000 to 1, so since you hold the max, there is a good chance that you will win some each month.
The machine that selects which bonds win each month is called the Electronic Random Number Indicator Equipment (ERNIE). It just picks random numbers, so if it's fair (which we have to take on faith), the time you bought it or the block structure of your bonds does not matter.
Good luck on the big prizes being awarded this June!
I hope this helps. Let me know if you need more information.