Runoff is accurate enough.
pH 6 is too low, and if you are at the bottom of the range that your strip says, how do yo uknow it's not actually lower? Get it up between 6.5 and 6.8.

If you are using enough perlite or vermiculite, flushing should not be a problem. Use water that is at pH 7 since your pH is TOO low atm. Flush with 3x the volume of the soil. This sounds like a problem of lock out and once you get your soil pH up to the 'good' range you will see improvement.