Thumbnails not appearing as expected

[caflynn]

Describe the bug
A couple of different issues have been reported regarding thumbnails. For example, the UO Communications officer reported that this item had an incorrect system-generated thumbnail-- so, for Winter 2018 issue, an image displayed, but it was not the one in the PDF. I replaced the thumbnail file with an image that matched the cover of the issue, but it does not display.

To Reproduce
I think it could be reproduced by removing and re-adding a thumbnail. I am reluctant to do this myself and cause more items to appear without thumbnails.

Expected behavior
A clear and concise description of what you expected to happen.

Related work
Link to any related tickets or PRs here.

[caflynn]

Thumbnail issue noted by submitter Christy Carmichael, re: Missing thumbnail from https://hdl.handle.net/1794/29874 . The image displays in the full view of the item, but does not display from the browse list for the collection: EWP Fact Sheetshttps://hdl.handle.net/1794/19407

[caflynn]

Another detail reported is that there seems to be a particular issue with a "replaced" file. Occasionally, we need to replace a bitstream-- sometimes there are errors in the first file and it is replaced at request of the author, or sometimes the wrong file is added and needs to be replaced. The ticket referenced above is one such example. I have not been able to discern if there is anything different that would prevent it from appearing-- just that it is a replacement.