Hey guys, so I was in the similar situation, where my SK mesh wasn’t showing up either, i had to count manually the number of bones i had ( I know there might have been an easier way to figure it out… but i am a lil old school). And the solution, I found is that you have to delete the bones from your skeletal mesh in Maya (not in unreal) and make sure that they are under 75 as some one had already mentioned it. after you have done the changes, make sure the bones that you have deleted are the important ones (try deleting the ones that have no animations and no paint weights, like the tips of the fingers) and after, save the file with the same name or different name ( your choice) and re import the skeletal mesh in UE4. and that should do the trick. Hope it helps.
P.S. i know its a little more extra work… but hey at least this method works.