Change order of activation for ad blocks on the page

Hi,
Here is the problem. There are two regions in the right-side column of the website - r1 on top and r2 just below it. 3 ad blocks: r1a1 in r1, r2a1 and r2a2 in r2. Only 2 ads - a1 and a2. Both ads assigned to r2a1 and r2a2, that makes them to cycle their places: on one page a1 inside of r2a1, and a2 inside r2a2, on the other page a2 in r2a1 and a1 in r2a2. Block r1a1 has only one ad - a1 assigned and it's called only on specific pages to make a1 be always on top of a2 for those pages. But what is happening - blocks in r2 are firing before the block in r1, showing ads at random positions with block in r1 collapsed (empty as each ad will only display once for the page), as a1 and a2 already initialized for the page.
Is any way to change the order, so r1a1 will be activated before r2a1 and r2a2?

Thank you so much,
Gennady