55G's is a pretty large tank, sometimes i have trouble finding maybe 1 of my 3 mystery snails in my 20G. they are usually hiding the plants (if you have any) inside or around ornaments, or can even be hanging upside down on the hood. just this weekend while i was gone, one of mystery snails escaped my tank because my sister left the hood slightly open after feeding the fish. he fell, sort of cracked his shell and rolled all the way to my bed a couple of feet away. fortunately my sister found it on the floor and placed him back in. after unraveling himself and releasing stuck air pockets in his shell, he slithering on though i hope it's shell heals. your snail might just be hiding.
