Yesterday WWCA Buy at Close order filled at 4.9975. Look at the last minute on the attached chart. Is it normal ? This would mean that at close orders can not be used.
Market on Close work well with NYSE stocks as there is one closing print. You on the other hand were submitting a MOC for a NASDAQ stock. Since there is no closing auction for NASDAQ stocks, the order is simulated and sent to an exchange or ECN at 4PM as a market order. IMO, your best off submitting a limit order yourself for NASDAQ stocks.