Thumbnails display works inconsistently in Views query

How are you, all? My case is not always bad. I am using a image field to upload images and those are being stored in sub-folders in private folder. The thumbnails displayed in the nodes are always fine and also their original sizes after clicking to enlarge. The only problem is the display after query sorting by the views. Most of the time, 90 of 100 times, the thumbnails in the views are fine. I mean all the thumbnails are displaying normally in majority cases. The ten is that thumbnails are not able to show normally. Sometimes thumbnail A works fine but sometimes not. As well, B works well but another sometimes not. In these abnormal cases, there maybe half amount of thumbnails showing fine but another half not.
If anyone can help, it would be very appreciated.
Recently, in these weeks (after upgrade to core 7.14), all the times, 100 of 100 times works bad (previous 10 of 100 bad). Views only show half of the total number of thumbnails and every time different. Please see attached 1 screenshot and 1 pdf file.
Remark: another drupal sites I am storing the images in public folder are always working fine. Never with this problem.

I have made a bug report in view project but it was said not related to view.

*** A view contributor, dawehner, replied me like that:
I reallly doubt that this is something caused by views, as views just renders some html.
The actual picture is generated on a seperate request by the image subsubsytem, so private files may cause the problems here. See the node link: http://drupal.org/node/1614580