We have a new release of Agenda in beta testing now. We will be starting a broader test in a few days. It does include a few sync improvements. If you are interested, you could try that out. If you want to join the beta, post a comment in this thread: Can I join the iOS Beta?
If not, TBH I would probably try iCloud again. In general, it is more stable in Agenda than Dropbox.
Do you have a lot of notes?
What I would probably do is turn off the sync inside of the Agenda app on each device, and when it asks, remove the cloud data. That removes the data from Dropbox.
Then switch to iCloud inside of Agenda on one device, leave it for a minute or two, and turn it off again. Again, choose to remove the cloud data.
Finally, turn the iCloud sync on in Agenda and wait for it to finish. Then try it on the other device.
Best not to continually change the sync setting. That can make things worse. If the cloud is flashing, it is still busy syncing.
You should also check in Settings app if you are signed into iCloud with the same account on each device, and that iCloud Drive is turned on for each device.
Kind regards,
Drew