One thing to review is the spacing between your cadence sensor and smallest gear (11-tooth cog). This spacing should be less than 2mm, but can be affected by the order in which the items were installed or if something has come loose. The cadence sensor will measure the change between a passing tooth, but will only work at a proper distance (less than 2mm).
You may use a nickel (5 cent coin) to perform this test, as a nickel’s thickness is about 2mm. If it slides between the cadence sensor and the smallest cog, the space is too large and should be adjusted. See the following diagram and video