It was brought to my attention by the 1st base coach that the opposing pitcher's pivot foot was coming off of the rubber about an inch to an inch and a half during his delivery. After a couple of pitches I called an illegal pitch and informed the pitcher to try and remain on the rubber during delivery. During the half inning I examined the mound with the home plate umpire and we both determined that the pitcher's foot was sliding into the "toe hole" that was made by both side's pitchers. No additional illegal pitch calls were made the rest of the game due to the wet conditions and both teams constantly "in the hole". Should I have called an illegal pitch every time or just let the pitcher's pivot foot be good enough by being "in the hole"?