Are there fish in the tank? You don't want to play with the Ph and soda if you do have fish. One of the reasons for Ph drop during cycling is the high ammounts of ammonia that need to be processed which naturally acidifies the water. Out of curiosity but how much ammonia are you dosing, how often,what are the exact readings and when did you start the cycle?
I'd do a large water change again and if it's a fishless cycle, you can add soda bicarbonate to keep the Ph high for the cycle to finish. There is almost no cycle going at Ph of 6, the bacteria gets inhibited at this value and the cycle may stall.