Previews not loading for locally stored album Lightroom for android

I'm using lightroom on 2 android devices and both have stopped displaying many of the previews for locally stored pictures, some are showing but most of those imported over the last few days are not.  If I click on the blank image in the grid it will download the preview but it does not keep it, ie if I come out of edit it is still blank in the grid.  I have tried de-selecting "store locally" and clearing the cache then downloading the album again (time consuming as there are more than 700 images) but this has no effect.

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