As all of our races are based on live events, from time to time, the event that a given race is matched up with will "close" a pool.   This is generally done because there are not enough competitors in the race to justify the pool (If a race only has 4 or 5 runners, the chances are slim of missing your show wager) and therefore that pool will therefore get closed by the live track.
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If this happens, I'd recommend making a "place" wager instead - with the fewer participants in the race, your likelihood of hitting your place wager increases!