The issue is discussed in https://groups.google.com/forum/?hl=en-GB#!topic/drools-usage/aZU5mXSgHMI.

Here is the gist of it....

I changed the accumulator to the following and it worked:

accumulate($sm:StatusMessage(campaignName==$campaign.campaignName, status==ManagerCallState.CALLER_FAILURE, attempts<3, sent==true), $message:oldestRetry( $sm ))

The previous approach was this:

$message:StatusMessage() from accumulate($sm:StatusMessage(campaignName==$campaign.campaignName, status==ManagerCallState.CALLER_FAILURE, attempts<3, sent==true), oldestRetry( $sm ))

which I thought was the correct or acceptable syntax option. Is there something I have overlooked in the second one ("from"??). It does give me a StatusMessage object but it lacks the underlying properties to allow the subsequent modify to work.