It's understandable that pH can be a little confusing. A couple of things to check..
1st, is your pH pen in good working order? When's the last time you replaced the batteries? Pens do die if your dry out. It changes the chemistry within, and they'll read wrong.
2nd, Did you use you your device properly? I'm not sure of your experience level, but using pH testing equipment will get easier with more experience.
3rd, Is your pH solution in good standing? Where most Acids won't go bad, per se, they do get contaminated.
4th, and last, the Yoyo effect. As Acid disolves carbonates within a solution, the results of the process will raise pH again (i.e. Add acid = lower pH + disolved salts = pH back up). This yoyo effect will continue until all the carbonates are disolved.
Once all the carbonates are completely disolved, pH will drop sharply, so be careful not to overdose with pH down.
Here's a video that might help.