Among the problems the FEers have is that the evidence of the 20th century is working against them. On a perfectly flat surface - such as one of reasonably limited dimensions such as a basketball court floor - you could see, with your eye or a mirror right at the floor, all the way across to even confetti resting on the floor on the farthest end. But in terms of serious distances, someone whose head is merely above water in the ocean could not see a beach more than 3 miles away (he might see the tops of trees that far away though). Even going to the Observation Deck of the Empire State Building will not enable someone to see Trenton NJ or Boston. Even a jetliner over NYC will not have a view of Los Angeles. The FEers have strange explanations for this involving optics - stuff Newton never said - but the simple fact is that the higher you go the more of the Earth you can see, which is consistent with a globe and, in fact, by doing some calculation about the altitude and how far the horizon reaches, you could make a pretty fair guess about the size of the globe.
That's one. Another problem is that, even now in the 21st century, the FEers have yet to come up with a convincing map of the Flat Earth. It shouldn't be that hard. All the conventional map makers can come up with road maps that pretty much are confirmed for mileage and direction, so why do we doubt them when they get ambitious enough to map the whole world at once? So far I'm not even sure an FEer could draw me a map to the neighborhood grocery - they have yet to come up with definitive map of the planet, showing the EDGE (and it should be in every direction if you go far enough). They come up with wild stories about Antarctica, which we now know aren't true, but after decades of propaganda - nothing solid, not one intrepid explorer from the FES has even tried to reach the EDGE in any direction. They might as well be telling us that Jack's beanstalk is still reaching into the sky somewhere.