I have not heard of many one egg clutches... if any at all.
How often are you checking in on her? I would leave the lay box in there but make sure the snake is not disturbed. Check for eggs once a day and not more than that. Laying one slug first and the rest of the clutch later is quite common (I've even had it take up to a week after the random slug or two shows up) but the snake needs to feel the lay box is a safe secure place before she will lay her eggs in it.
Also how long ago was it that she shed?