Medela pump parts

I have a Medela double electric breast pump that I use four days a week at work and occasionally at home for 5 months now    I used to have an oversupply so getting a lot of milk was easy. Now my supply has stabilized and it takes longer. I'm not sure if it's because my boobs aren't rock hard all the time or if it's just use, but the suction doesn't feel as strong anymore. I've read in mommy blogs about people taking about replacing parts every 3-6 months....what should I be replacing exactly to make it function better?