Ian,
You may need to have the logs on your controller analyzed so you can get specific information about your environment. Very few environments require changes to the ARM parameters, but when they do, it is usually done with the engagement of support to get the desired results. In the 2.4 ghz band, if you have a medium to dense deployment with poor isolation there are very few choices which channels APs can be deployed. If you are dense enough, access points in the next room could certainly end up on the same channel, because coverage is not the only thing measured when choosing a channel. it is also quite possible that an AP can see another AP above or below it stronger than the one next to it, and that will affect its decision, as well
Please contact support to ensure that your config is optimized for your specific situation.
Please also take a look at the knowledgebase article about ARM here: http://support.arubanetworks.com/Default.aspx?tabid=111&loc=https://kb.arubanetworks.com/app/answers/detail/a_id/927