DWC Northern Lights, problem with ph dropping low

I am on day 59 of flower and recently started getting this issue. I just changed the res 3 days ago, ph was around 5.8-6.0 until this morning when it had dropped to 5.3, tried adjusting with ph up but not having any effect

I had this issue last week as well, so decided to change the res - do i need to start changing the res every couple of days now instead of every 5-7?

I have checked the root ball and i don't see any slime or notice any funky smell. It's not ready to harvest yet so wondering if anyone has experience of this issue, is it just end of flower stuff? This is my first DWC run

thank you

There is a reason for the ph slip.
Idk what it is.
I Like my ph around 6.2 at day 59
The plant has better uptake at that ph
5.8 is for veg.
your at day 59...it doesnt matter that much now.

Just a note for next time...
Switch your ph with your plants flower period.
5.8 >>>>6.2
You will see the difference

I use a ppm meter to check exactly what ph the plant was eating at....

If i start with 1600ppm
I want to see 500ppm at rez change

Where did you get the NL from....
Is it NL5 or NL2

I am no DWC grower but I assume that the plants have consumed more water than nutrient, hence the PH drift upwards since most nutrient are acids.
Did you check the EC?