I am not in front of my H8000FW to test this, but make sure that the LFO has a rate of something like 1-2 Hz and that the decay in the reverb has something like 500 (default value is 1 as I can see, which is a very small value given the fact that the range is [1..1000]). If it doesn't do anything, you may want to multiply the depth of the LFO so that its effect is more prominent when it's doing its thing.

Other than that, modulating reverb decay is a little weird, in the sense that the results might not be easily distinguishable. Try sending the same LFO to a cutoff, just to make sure it's working properly.