Soil meters are virtually useless in a cannabis garden. Very unreliable, and all you are checking is the ph of the buffers in your soil at the point the probe touches. Stick it in 10 times, likely 10 different results. Usually the buffers in a commercial mix last about 4-8 weeks of heavy use before they dissolve away.

Your ingoing water ph is very important though, as is the runpoff ph.
At the very least, go get a freshwater aquarium ph test kit, ($7.00 US) and check the ingoing ph of your water. Or take a cup of water to your local aquarium shop and ask what the ph of your water is. They'll usually test for free. Don't ask a friend with an aquarium unless prepared with a likely excuse.

Plants looked ok though. No obvious signs of stress.

Vinegar or lemon juice as a temporary measure only, but you'll need to test for ph to know if you are on the right track. (for how much juice to add)