Post Indexer not indexing all sites in network


The plugin been installed for a week or so now and it still hasn't indexed all the websites in the network, we only have 19 sub sites why is this taking so long? I have rebuilt the index a number of times and also clicked to do this on each individual site but it still says there are 19 sites in the queue. Do you know why this is happening?

Many thanks,

James Isles