She could have infertile eggs as the others suggested. As sand is a poor medium to lay eggs in, since it can't hold it's shape well or long, I suggest picking up a rubbermaid container about half again the length of your dragon in length and half your dragon's length in height and depth (as an estimate). Fill with non treated earth (potting soil is fine as long as t here are no chemicals, insecticides or fertilizers in it) and mix with vermiculite. Add enough water to completely dampen the mix (top and bottom) so it's damp enough to clump down and hold it's shape but not so wet you can squeeze water out of it. Put a regular basking light over the soil to warm it up and put your dragon into it and leave her alone. You can cover part of the container as well for privacy. You also can dig a partial hole for her as well.
Leave her in the egg laying bin for a few hours (you can put this into her regular cage completely, if there is room etc). Take her out if she stops digging completely and just sits around ontop for any extended period of time. Every night check and see if you find any eggs, if you do, tosss t hem out and your dragon should be fine. If you don't find eggs in the soil you can put the dragon in again the following day.
If she never lays eggs after a week but still shows digging behaviour...it's possible she may think it's time to brumate and is trying to dig a nice safe den to use.
If her health declines in any way, or she stops eating (note that she may do this just before laying eggs too but normally only for a day or two before laying eggs, then she will be ravenous once she is done). Or if she shows any signs of stress or listlessness, take her to a vet to have her checked for retained eggs (ie eggbound).