It should have been one of the first things they checked, but mechanics are like doctors, 50% of them are in the bottom half of their class. If they changed the pump out, and you still have the problem then that was not the problem. A good mechanic will diagnose the problem first then fix it, but also like a doctor, the early diagnosis can be wrong because they are trying to find the problem quickly and some problems are difficult to diagnose.
For amateurs like us, we diagnose things and then fix the cheapest and easiest stuff first before going to the costly and difficult.