Can someone help

It could also be nute burn, nute deficiencies, heat/light stress, too little light/heat... any number of things really. If the yellowing occurs at the bottom, it's likely nitrogen or the bottom isn't getting light. If it's at the top, it could be a micro deficiency. If you posted pictures and most people are saying pH, then calibrate your meter. You'll need buffering solution and a measuring cup that measures in mL. You can store the solutions in Mason jars and they should be good for 10 days or so. If you're using tap water, let it sit out for 24 hours before watering with it. Good luck!

If you keep having problems in soil.. Test water run off. Generally speaking I do not like to water to run off... But this would be a case where seeing what comes out is important... You test both your E.C. and your PH of your run off.. But that's if your trying to diagnose. to bad you don't have a diary because it's a lot easier to pin point exactly what's wrong.. Like others have said it could be another unknown thing we aren't considering.. Pictures are worth a thousand words.

Have you tried mixing up a calibration solution like 6.86 and put your pen in to see if it gives you the correct answer? If it doesn't then you need to recalibrate. If you're using the strips...those are a little bit open to interpretation so they can be off sometimes because of color matching. Also...sometimes it may look like it's a ph problem but could be down to a lock out from too many nutrients, or the soils all unhappy muddy or something so that'll make nutrients unavailable too.

https://www.amazon.com/Digital-Measurement-Household-Drinking-Aquarium/dp/B0894LBJL3/ref=sr_1_4?crid=VZLDCP48N4SD&dchild=1&keywords=ph+meter&qid=1595475856&sprefix=ph+%2Caps%2C482&sr=8-4