PLEASE HELP.

Yesterday I spent 2 hours trying to fix my PH and could not reach what EVERYONE say.

The following tests were made :

Made my water PH 7.0
After giving the plant water, I collected the first drops from the runway and tested them, It showed PH 4.5.
I added PH up, made my water PH at 9.0 so that the runway becomes 6.5
But after watering and collecting the drops from the bottom, I tested again and it showed 4.8, how could that be ?

I made that test again on a different plant which needed to be watered and same results.

So how could I fix this ?