Please use this troubleshooting guide to help you resolve known issues that may occur when using Media Manager.
Uploaded Media Not Showing In Media Manager Library
This issue mainly affects users of ProcessWire 3.0.154+
This issue is related to an error you would see regarding WireDatabasePDO. If logged in as a superuser and config->debug true, you should see the message. In short, a recent change in ProcessWire has broken Media Manager. A bug report has been filed for ProcessWire. Due to the error, the media creation process shuts down, hence, the 'images/files are not visible' inside your Media Manager Library. In fact, you will most likely find the images in one of the folders inside /site/assets/MediaManager/jqfu/.
As we wait for a fix in ProcessWire, a workaround is to go and edit each Media Manager-related field listed below and check in theDetails Tab, under Use Tags that 'User enters tags by text input' is enabled. If you find it is already enabled, disable it (Tags disabled). You will then have to save the field. Now re-enable tags ('User enters tags by text input') and save again. You might have to save twice or thrice for the tags setting to be correctly set by ProcessWire at the database level.
Media Manager fields to edit:
Due to the failed media creation, you might find that when finally Media Manager works, you might get notices about duplicate images/media (if you had previously attempted to upload media). These would be orphaned pages. You can manually delete those pages under admin > Media Manager > Media Manager: Image. Alternatively, properly uninstall Media Manager and reinstall it but before uploading images, work on the tags issue as instructed above.
If the above steps still do not work for you, you are probably experiencing a different issue such as restrictions on files upload sizes. In this case, please contact us for further assistance.