HR 1217 is a prototypical rapidly oscillating Ap star that has presented a test to the theory of nonradial stellar pulsation. Prior observations showed a clear pattern of five modes with alternating frequency spacings of 33.3 mu Hz and 34.6 mu Hz, with a sixth mode at a problematic spacing of 50.0 mu Hz (which equals 1.5 times 33.3 mu Hz) to the high-frequency side. Asymptotic pulsation theory allowed for a frequency spacing of 34 mu Hz, but Hipparcos observations rule out such a spacing. Theoretical calculations of magnetoacoustic modes in Ap stars by Cunha (2001) predicted that there should be a previously undetected mode 34 mu Hz higher than the main group, with a smaller spacing between it and the highest one. The 20th extended coverage campaign of the Whole Earth Telescope ({XCov20}) has discovered this frequency as predicted by Cunha (2001). Amplitude modulation of several of the pulsation modes between the 1986 and 2000 data sets has also been discovered, while important parameters for modelling the geometry of the pulsation modes have been shown to be unchanged. With stringent selection of the best data from the WET network the amplitude spectrum shows highest peaks at only 50 mu mag and formal errors on the determined amplitudes are 14 mu mag. Some lessons for future use of WET for the highest precision photometry on bright stars are discussed.