No such thing as "one thing" that they struggle with the most. Everyone is different. Some folks are rock stars and pick up everything right away, some struggle with a few (or more) skills, and some find there is something that turns out to be a show stopper. Even among the ones that struggle at first, some catch on quickly after that initial struggle, and some continue to require remediation.
Now, regarding "show stoppers," aside from just overall anxiety prevents someone from overcoming their fears, I have most run into folks that just can't stand to have the mask off of their face in the water.
On other "common" issues, I find that when I notice a pattern of frequent difficulty that it can often be addressed by modifying the way that I am teaching, so that the pattern fades away. For instance, I began to notice some divers struggle with their first giant stride entries into the pool. (and that can be a whole process to have them repeat it - have to get out, get the fins back on, and try again without falling over!) I now have all of my students first enter the water with a giant stride wearing only snorkel gear, so that they can work on the mechanics of the stride before I add on all the weight and bulk of scuba gear.