First off, how are you measuring? Second. are you keeping track of what you are doing? (Writing down the amounts you add so you can adjust from there.)
Third, if you are measuring with an electronic pH pen, which one is it, and how much did it cost? Cheap pH pens (under $75 or so) are notoriously inaccurate.
Fifth, what are you mixing into your nutes, and what sort of water are you starting with? (RO, tap?) What is the EC of your mixes.
Sixth, what is the size of your reservoir? (I assume you have a reservoir feed system, but it would help to now how you get you water to and from the plants.)
I am also growing in coco, and when I mix my nutes, I tend to end up at pH 6.1, with a goal of 6.0. I use a
Bluelab pH pen to measure when I mix. (I mix 10 gallons at a time in a big tub, then pour into a 30 gallon reservoir. I mix strong, then add some RO water into the reservoir to get my EC where I want it
Once in the reservoir, I have a
Bluelab controller hooked up to pH DOWN to keep the pH from rising. I have noticed when the reservoir is about 1/4 full that it tends to have the pH move lower, to 5.9. I then add a half-capfull of pH UP into the reservoir. AND I WAIT 10 MINUTES before trying to adjust further. I also have a pump stirring the water and a bubbler in the reservoir to ensure it stays mixed up.
My reservoir is at 72F, which is supposed to be a bit high, but that's what it is. I don't try to cool it from there. The evaporation from the bubbler cools it to the level it is.
I'm thinking you are not measuring and dosing consistently, not waiting long enough for a stable result, and possibly have a dodgy measurement system. More than anything, I suggest waiting longer between adding doses, and adding half the amounts you've been adding.