Panning sends not working

I don´t understand what exactly you mean by “use the sends to duck the reverb just behind each guitar”, but if you want the reverb from the L 50 guitar to be at “only” L 50 and the reverb of the R50 guitar to be at "only"R 50, you will need to use two mono reverbs FX tracks, with the Returns panned one R50 and the other L 50.