@Maiaibing and @stuck could this be related to the PL6.0.1 bug reported here PL6 stops loading all previews halfway with 'loading please wait ...' and Win 10 DxPL6.0.1 Can't handle large directories - thumbnail refresh fails and elsewhere.
If you feel that you can move to PL6.1.0 then
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Secure a copy of the PL6.0.1 (presuming that is the release you are on), marking as the PL6.?.? release, before downloading the PL6.1.0 release.
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If you are processing any images by first putting them through the Adobe DNG convertor or otherwise presenting DNG image to PL6 then be aware that there appears to be a bug in DxPL6.1.0 with respect to such images!?
Please respond on whether you feel that is strategy might help
Regards
Bryan