Thank you very much, @chris , for the detailed information! Now I think I got it.
One additional note for others with the same problem:
after reconfiguring my affected devices (like @HomeAutomation suggested) and waking them up to receive the new parameter values, nothing changed in the behaviour which means I got no information about the battery levels of these devices. But as I saw in habmin that the device is now more frequently waking up, I created a second openhab-item for the battery level besides the already existent one … but with the difference that I did it by using the UI instead of defining them in an items-file. From then on, everything worked fine … even with the “old” items. Since then I get the battery state regularly about every six hours. Strange, but it works now!
Thank you very much for your support to all of you!