If there is a problem with writing, either reported by the server or detected by the client, the EasyUAClient.WriteValue should throw an exception (OpcLabs.EasyOpc.UA.OperationModel.UAException, actually).
If you make this call but the value does not change in the server, then I can think of following possible causes:
1) A problem in the server - it does not actually perform the write, but does not return an error to the client,
2) A bug in the client: it receives an error indication from the server, but does not throw the UAException as it should.
3) The UAException is thrown but your program catches it and "swallows" it silently (not likely but still a possibility).