A problem that may be causing this is when MacTCP is configured for Manual addressing and the MacTCP stack is initialized before the ARA connection is made.
This can be reproduced by installing MacSNMP software on a remote Macintosh. MacSNMP software will load at startup and initialize the MacTCP stack, causing MacTCP connections over an ARA link to not work. Removing MacSNMP software from the remote Macintosh resolves this.