Too much of the animal is being detected, preventing CatWalk from focusing on the actual prints, or display of the rectangles has been turned off.
If the Camera Gain is high or there is too much ambient light in the room, especially if the Green Intensity Threshold is too low, CatWalk may detect the abdomen or even the complete ventral side of the animal as a paw print. In this case, you'll see parts of the animal showing up in green, as shown here:
When this is the case and in the Acquisition Settings the option "Discard incomplete prints at the start and end of a run" is selected, the following happens: as soon as the animal enters the camera view, the ventral side of the body overlaps with the border of the camera image, and, because of the selected option, the whole body (that is, all the prints) is discarded.
To prevent this scenario from happening, deselect the option "Discard incomplete prints at the start and end of a run". You can access this setting by going on the menu to Setup > Acquisition Settings.
You may also wish to adjust the detection settings if the Camera Gain is too high or the Green Intensity Threshold too low.
Right-click in the orange (background) part of the image and make sure "Show Rectangles" is checked.