Hmmm...why would you want to open the radiator cap when the engine is running??

Normally, bukas na dapat ang radiator cap when you crank the engine to observe the water in the rad. It's a no-no to open the rad cap with the engine already on, kasi mabilis uminit ang makina (less than 5 mins).

But to answer your question as to why the water doesn't come gushing out when you open the rad cap, it's because there isn't sufficient pressure sa radiator (due to leaks sa cooling system) or the water pump is defective