Welcome to the GERD forum! Yes, people often find that a PPI becomes less effective over time. There are many different types...I started on Zantac 300, then was switched to PPI's when that wasn't doing the trick.
I've been through Prilosec, Nexium, and lastly, Protonix, which seemed to work the best. Also there's the dosage...are you on 20mg once a day? That can be upped to 20mg twice a day, 40mg once a day, or 40mg twice a day.
It's definitely time to contact your doctor and let him/her know what you're experiencing.
Good luck getting to the bottom of your problem!
Hope you stay with us! We'll do our best to share our experiences and support you in your search for answers.