Usually, this is simply a case of giving the device enough time to upload/download the data. You could hold Agenda in the foreground for a while, just stopping your device from going to sleep, to give it time. If you don’t want to to have to constantly keep the device from “sleeping”, you can turn off the Autolock in the Settings.
Repeat this on each device, letting it sync for around 20 minutes. Usually that will allow everything to upload and download, and it will be fast after that.
Note that you can see if it is syncing by looking for the cloud icon at the bottom of the right panel. It appears when there is sync activity.