Salt buildup from your nutes will cause your soil pH to drop. To fix this problem you should flush your plants with double the amount of water as there is soil, so if your in a 2 gallon pot your going to flush your plant with 4 gallons of water. The 4 gallons of pH adjusted water will remove alot of that salt in your soil, then you can test your soil run off again. You should be around the correct range, if not, keep flushing until you reach that magic number. IF you do not flush your plant, you will end up in a lockout and if left unattended could take you over a month to get out of.

Hope that helps.