STP blocks a link in a VLAN even though there are no redundant links in that VLAN

In 802.1Q-compliant switches, MSTP blocks redundant physical links even if they are in separate VLANs. A solution is to use only one, multiple-VLAN (tagged) link between the devices. Also, if ports are available, you can improve the bandwidth in this situation by using a port trunk. See "Spanning Tree Operation with VLANs" in "Static Virtual LANs (VLANs)" in the advanced traffic management guide for your switch.