Which pH pen do you have? And do you have calibration and storage fluids? I'm on my second Milwaukee Instruments pH pen. They are cheap and need a bit of regular maintenance but for $25 it works great. I spend more on cal and storage fluid initially but they all last a long time.
I recommend this guy:
The Milwaukee Instruments compact pH pen tester is often used to help maintain pools and aquariums as well as improve results for growers and quality in a range of food and beverage uses.
milwaukeeinstruments.com
My first one lasted 2.5 years, one battery change, and was still good but taking too long to read.
You can get them from Amazon for a few $ less but I ordered direct and it came with more in depth instructions.
No matter which one you get some "hidden" tips I've had excellent luck using:
1) Put fresh storage solution in the cap and let it soak overnight to fully hydrate or rehydrate the glass bulb before use.
2) Calibrate with both 7.0 and 4.0 solution
3) Recalibrate once a week when new, once every 2 weeks after that, and then about once a month ( I usually drop to only using the 7.0 cal fluid after the first 4-5 weeks if it looks to be holding steady)
Most of them have some break in time and doing the frequent recal is essential until they stabilize. Especially on the "cheap" end of things. I have considered the high $$ meters but eh, I'll do maintenance for now.
It is wild how much difference a little swing in pH can make.