Guys,
Maybe you are using a different latency.
Example: If you to use in G723.1 a latency of 30mileseconds you will to use about 16Kbps, but if you to use G729 with a latency of 90mileseconds you will to use about 12Kbps.
On the other hand if you to use G729 with a latency of 20mileseconds you will to use 24Kbps.
Maybe in your case you need to adjust the latency to get a good compression.
Good lock,
Silvio.