The single most common cause is dropping the file into a different Dropbox folder than the one DropPost is actually watching. Open Connect Accounts to see the exact watched folder path, and confirm your file landed there.
"Why didn't my file get picked up?"
If you dropped a photo or video into Dropbox and it hasn't shown up in your Post History, there are a few common causes worth checking in order before assuming something is wrong.
What you'll need
Access to your Dropbox account and your DropPost Connect Accounts page.
Steps

DropPost only picks up supported photo and video formats. If the file is an unsupported type, it won't be detected — try re-exporting it as a standard photo or video format and dropping it again.
Detection is near-instant in almost all cases. There's also a periodic backstop check running as a safety net, so on the rare occasion something is missed initially, it self-corrects within a few minutes rather than being lost for good.
If it's been much longer than a few minutes and nothing has appeared in Post History, that's exactly when to reach out. Use the Contact Support form on the Help & Support page.
What happens next
Once the file is confirmed in the correct folder and in a supported format, it should appear in Post History shortly, moving through DropPost's normal captioning and publishing flow.
