Communications problems?

The Siemens devices I use are connected, and I have created about 1800 tags. I have got a problem now. If I create a new tag, the value is not taken. The green circle with a question mark appears next to the OPC tag icon. I do not know what to do, please help me.
This happens also if I copy a tag which has its own value changing in real time. The new tag does not show any value.