If the problem persists after you change your pressure-reducing valve, then you either have clogged pipes or a leak. Neither scenario is good. It's time to call a licensed plumber and see what your options are.
The best thing to do if you think your water pressure is too low is to call a professional to assess your water system. Then you will know exactly what you can do and how much it will cost to get it done.
You might want to visit our Plumbing page for answers to your questions.