Check it out very carefully to make sure it is not leaking/spilling, as that is the likeliest explanation. The wetness will typically sink to the bottom of the litter and may not be obvious on the surface, but will cause humidity/mold/rot/etc problems if undetected and uncorrected.
One common problem is if the waterer is on a very slight tilt (with the opening through which the trough fills on the non-downhill side of the tilt)... it will very slowly overflow til all the water is gone. Or sometimes it is only tilted enough to overflow when jostled by chickens drinking at it. If there is any question, put it on a tray (or etc) of new dry shavings, to detect any spills, and to reduce the chance of the problem make sure that the hole is on the *lowest* side (that may sound counterintuitive, but that is the configuration that reduces tendency to overflow)
Good luck, have fun,
Pat