Once you thread is waiting for the Mutex, another thread must release it. Then the tasks waiting for it should be put into the state READY (http://www.keil.com/pack/doc/CMSIS/RTOS/html/group___c_m_s_i_s___r_t_o_s___thread_mgmt.html).
You can verify that using a debugger and the OS Kernel Awareness information. You may contact our support team http://www.keil.com/support/contact.asp, but we would need to understand the relevant portions of your source code to help you.