Excessive amounts of baking soda needed to raise TA

Following a few weeks of heavy rains my TA dropped to 30. I then started adding 5# at a time of baking soda every few days but each time the TA increased by only 10 but according to poolcalculator.com it should have raised it over 20. After 3 lots of 5# Baking soda my alkalinity is still only 60. I have added BS in the past in accordance with the recommendations given by pool calculator and always achieved the results expected.
My pool is inground 13000 gal and I use a Taylor test kit. The PH is high at 8 but I have not added acid as I was waiting for the TA to reach 80 before doing this.
Why would I now be needing over twice the recommended baking soda???