used 20lbs for 1200sqft. checked ph after 3 weeks, no change. not enough lime?

Hello, 15lb per 1,000 sq ft would be an appropriate amount of product if your starting soil pH was less than 5.5. We do not recommend applying more than this amount per application. 3 weeks is the expected duration for the granules to fully dissolve with rain/watering. It is possible that the product has not yet had enough time to work its way into the soil. Do you happen to be dealing with clay or compact soil? These conditions can lengthen the amount of time it takes for Fast Acting Lime to be effective.