Simulink "Real-Time Synchronization" Block Not Working Properly On Some PCs

I need to run some Simulink models in real-time while in Normal Mode. I tried using Real-Time Synchronization block and it worked perfectly on my Laptop (Win7 x64, Core i5 CPU, MATLAB R2014a) & Another PC (Win8.1 x64, Core i7 CPU, MATLAB R2014b). But on a couple other PCs with almost similar specs, it does not synchronize the model with the real time, i.e., the simulation time runs much faster than the real time. I noticed that I had not installed the Real-Time Windows Target ('rtwintgt -setup' command) on the problematic PCs. After installing it, the models start to lag behind the real time. What is causing this problem?