We use adsense synchronous ad units on a site I'm involved in without any issue. Only 3 units per page. Site has a set of (3) major pages which are dynamically generated.
One 2 of the three, we changed to asynchronous ad units and found the following
Page 1 (only 1 ad unit) - does not appears - switch back to synchronous and ad units immediately start appearings
Page 2 (only 2 ad units). 1st ad unit shows ads but 2nd does not. When switching back to synchronous - both units show ads.
We've waited several hours with no change in this behavior. Checked the browser console and no evident errors are found.