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strange, the thumbnails was failed to be created. could you please asking that users to delete and re-upload the pics?
I tried to reproduce the issue at your site. but cant.
not sure that can happen.
if you compare both folders
[joomla site root]/images/photos/4595/2070
[joomla site root]/images/photos/4000/1865
one folder having thumb files and other one not. that because in some reasons the files are missing/not created at the server.
thumb always created at server side. no other why, that user must re-upload the photos again.
until now jomsocial doesn't have tool to re-create the thumbnail.