I have two reports which are identical with the exception of the earliest modifier. One has earliest="8/22/2014:00:00:00" , the other earliest=-6months. (The names of the reports are also different.)
The former works as expected. I can't get the latter one to accelerate. In Report Acceleration Summaries the one that works says "Pending Updated: 31m ago" and the one that doesn't work says "Building summary - 0% Updated: Never" and that status never changes. I've tried the Rebuild option under the Summary ID and also the Rebuild option under the Normalized Summary ID, but can't seem to get it to work.
There are several valid reasons that this could happen.
If the search returns less than 100K events, Splunk will not create the acceleration summary - it's faster for Splunk to do the search as needed. If the number of events grows to greater than 100K, Splunk will then create the summary. I think this is the most likely reason.