By way of explanation, when you send a command to a group, it gets passed on very quickly to each member. There’s no orderly queue. Especially when the Items are associated with real devices, you’d be dependent on the binding managing some kind of strategy of queue and delay. Some connection technologies simply aren’t very good at dealing with a flurry of commands.
To distinguish cases like that - OH command is okay but never arrives at device - from cases like you first described - OH rejects some command types - it is important to look in events.log to see what is happening with OH Items.
In this case, because you have a mix of Dimmers and Rollershutters, Udo’s solution is perfect.
But what if you have really different Item types, like Switches and Location? In this case you would not be able to send the same command to each through the Group at all. In that case you can do something like:
Grollo.members.filter[ m | m instanceof SwitchItem ].forEach[sw | sw.sendCommand(<switch command>) ]
Grollo.members.filter[ m | m instanceof Location ].forEach[ lo | lo.sendCommand(<location command>) ]
Obviously replace the stuff in < > with whatever is appropriate.