Server availability index is low on systems where memory, processor, and network are all reported as plentiful. (e.g. 1-15% processor peak, 20% network interface(s) peak, and "plentiful" memory).
Availability Index goes down to 0. Users may or may not see access issues. In addition, cluster failover doesn't occur (fails) since both machines are reporting 0 availability index.
Adjust the "Server_Transinfo_Range" from the default 6 to something higher (e.g. 8 or 10). Starting in Domino 6.5 and Domino 6.04, the documentation was improved with more information on these settings. See "Server Availability Index" in index of Domino Admin 7 Help.
The command sh ai will help you determine the proper values. I generally go one or two under the value unless I am really sure it's experiencing the maximum load.
A higher Server_Transinfo_Range value means a higher availability index (than 0 hopefully) when the server is under a decent load, and 0 when near or at it's peak/heavy load that would cause concern.
Remember that a server also has to handle requests that fail-over from another server in an outage, take this load into consideration when both planning your servers as well as for this index for what is truly a heavy/max load.