Post Indexer not picking up new posts...

The cron log shows things like:

Post Indexer First Pass
Finished First Pass Cron 	2013-06-16 12:08:07
Post Indexer First Pass
Running First Pass Cron 	2013-06-16 12:08:07
Post Indexer Second Pass
Finished Second Pass Cron 	2013-06-16 12:08:07
Post Indexer Second Pass
Running Second Pass Cron 	2013-06-16 12:08:06
Post Indexer First Pass
Finished First Pass Cron 	2013-06-16 12:05:36
Post Indexer First Pass
Running First Pass Cron 	2013-06-16 12:05:36
Post Indexer Second Pass
Finished Second Pass Cron 	2013-06-16 12:05:36
Post Indexer Second Pass
Running Second Pass Cron 	2013-06-16 12:05:36
Post Indexer First Pass
Finished First Pass Cron 	2013-06-16 12:00:13
Post Indexer First Pass
Running First Pass Cron 	2013-06-16 12:00:13
Post Indexer Second Pass
Finished Second Pass Cron 	2013-06-16 12:00:13
Post Indexer Second Pass
Running Second Pass Cron 	2013-06-16 12:00:13
Post Indexer First Pass
Finished First Pass Cron 	2013-06-16 11:56:20
Post Indexer First Pass
Running First Pass Cron 	2013-06-16 11:56:20
Post Indexer Second Pass
Finished Second Pass Cron 	2013-06-16 11:56:20
Post Indexer Second Pass
Running Second Pass Cron 	2013-06-16 11:56:20
Post Indexer Aged Posts Tidy
Finished Cron to tidy up Old Posts 	2013-06-16 11:56:20
Post Indexer Aged Posts Tidy
Running Cron to tidy up Old Posts 	2013-06-16 11:56:19
Post Indexer Postmeta Tidy
Finished Cron to tidy up Postmeta 	2013-06-16 11:56:19
Post Indexer Postmeta Tidy
Running Cron to tidy up Postmeta 	2013-06-16 11:56:19
Post Indexer Tag Tidy
Finished Cron to tidy up Taxonomy 	2013-06-16 11:56:19
Post Indexer Tag Tidy
Running Cron to tidy up Taxonomy 	2013-06-16 11:56:17
Post Indexer Second Pass
Finished Second Pass Cron 	2013-06-16 11:53:39
Post Indexer Second Pass
Running Second Pass Cron 	2013-06-16 11:53:39
Post Indexer First Pass
Finished First Pass Cron 	2013-06-16 11:53:38

But unless I manually rebuild the index new posts don't get pulled into the list.