Female Turtles can lay eggs without ever seen a male turtle. It is like chicken, who could lay eggs without "a chick in it".
If they do not have chance to lay this "empty" eggs, they will become ill, eventually until death.
As I know it, the turtle want to choose the time and place at its own. She should reach the sand location at her own. You must not to move her there. This could be difficult, but here you can see some examples, how it could work. (Site is in german language, but all links in the pointed list show pictured solutions for sun and sand places. For example try "Eiablageplatz" means "Egg laying place")
(My own yellow bellied slider do absolutely not accept it, if I catch it from the water. It is like a big predatory bird would take him as dinner...)
Another tricky thing is the composition of the "sand". For example if it is dry, the turtle can not dig a hole, because the sand run back in the pit. To mix sand with soil is a solution for this. You can test the mixture, if you try to dig with a spoon in it: it should be easy to dig, but the walls of the hole should be stable.
The location of this place is essential too. Your turtle is a smart one. She "feel" the water high, and choose a place that lay over the water, so her eggs do not become wet.
Another point, because you write "one of my turtles": Your turtles, like mine are the most of the time "loner". This means, they stay alone until they want to match and make little turtles. Because of that it will be tricky to grow them all together up. If they are grown you need a lot of place if you want to keep all together. For example everyone needs a sun place because the "bigger" will chase the smaller of it. But this turtles want to stay at the sun place as long as they could... so the smaller in fact can't use the sun place ever.