Quote from speculatus:
NYSE rule 123C:
In order to minimize excess market volatility that may be associated with large-size MOC orders that are entered near the close, and to allow sufficient time to attempt to offset large imbalances of MOC orders, there is a deadline of 3:40 p.m. for the entry of all MOC orders in all stocks on all trading days except for those orders entered to offset imbalance publications or on either side of the market if a regulatory halt is in effect at 3:40 p.m. or occurs after that time... Between 3:40 p.m. and 3:50 p.m., MOC orders are irrevocable, except to correct a legitimate error (e.g., side, size, symbol, price or duplication of an order), or when a regulatory trading halt is in effect at or occurs after 3:40 p.m., or to comply with the provisions of Rule 80A....
If you think you act according to these rules, but still have any problems with submitting your MOC orders, consult your broker.
BTW, if an exchange member (your broker) routinely violates this rule and submits wrong MOC orders before the close or market orders after the closed, NYSE will reject those orders and my fine the broker. It did happen in the past...