Common problem, not so easy fix. Older toilets flush a lot of water, but do it slowly. Newer ones flush much faster and the water can skip across to a back-to-back toilet. This can slosh things around, and lower the level. The fitting in the wall worked with the slower flushes, but you need a different one in there to work with any of the newer toilets. The Toto installation and spec sheets warn about this, and I'm sure other companies would have the same problem - they may or may not mention it in their documentation, though.