After successful 802.1X authentication, the client will initiate its DHCP request. Since you say it works with Windows 7, it sounds as though you have your VLANs and DHCP working. Any chance that Windows 8 client has a static IP set on its wireless NIC?
Two things to try/troubleshoot:
1) Try connecting this Windows 8 system using PEAP-MSCHAPv2 and see if it can get in that way.
2) Connect it to another network (Open or PSK). When you see the client in the user table, select it and choose "debug". While debugging, have the client connect to the 802.1X network using EAP-TLS and post the resulting logs.