The version you are running at the 204 is 6.4.3.1-4.2.x. The 303 is not compatible to run this software. Because of this the cluster is not formed correctly.
Please upgrade both devices to release 6.5.4.12 (this is the latest release where both IAP's are supported).
Because the IAP's are not the same hardware you should upgrade both devices manually. To do this, first power on and upgrade the 204. After this shutdown the 204 and power up the 303 and upgrade to the same release.
After the upgrade the IAP's should be able to join the same VC