If this setup is ok, then how many unique hits have you studied. Since both IIS servers point to the two servers in the cluster in the same order (WLS1:port,WLS2:port), both will forward the first new request that they receive to WLS1 (the first two new requests will go to the same server), the next two will go to WLS2 and so on.
If it is still being sent to the same server after several NEW requests, then something is either not setup right or it is a bug.
Giri