If you are checking after using the toilet, the liquid sloshes around and the sensors will pick that up. The smaller the tank the worst it is. I've found mine works great if I check before using the toilet/sink, half hour should be enough time.
If that's not the case, I'd be cleaning the tank (as mentioned above by others) and check the wiring to the sensors if you can access them.
SeeLevel tank monitors seem nice, problem is they require a fairly deep tank (I think 8" of level vertical surface). If you can fit the, and the cost isn't an issue for you.. probably worth it..
And as noted smkettner, 3/4 and full are probably the most important ;-)
