Yes I think it may be your banner is too long. What you can do is put some precommands into the device template and this help NCM wait a few more seconds before it trys to log in. Here is an example from the manual:
Telnet to your device to find the pre-commands you need. A pre-command can be used for any device which requires input before prompting for credentials. A pre-command is used before logging in. For example, when you connect to a router and before you are asked for a password, you must press
Enter
to wake up the connection. Add the following line to the template:
<Command Name="PreCommand" Value="${CRLF}"/>
.