WCF CallBack Blocks other Callbacks

Question

I have a WCF Service which Communicates to multiple WPF Client Applications. The Service uses NetTcpBinding in duplex Mode. Now I have 2 Functions which are called from the Server to notify each Client. One function is called every 50ms with a little amount of Data. (This function is invoked for every Client in a different Task!). The other function is called once, when the client registers on the Backend. This function submits a large dataset to the client.

The problem now is: when the function with the large Dataset is invoked (for one new client), on all clients you see, that they don't get data from the function with the LittleDataset. When the large call is finished everything works as expected.

Is there any way I could solve this? What more infos do you need?

ServiceMode=Session, Concurrencymode=Multiple, UseSychronisationContext=False