bug in backtesting

I've found what I can only assume is a backtesting bug (not tried this live trading):

In the code snippet below, you can see that my ExecuteMarketOrder methods are called inside if statement blocks. The issue I'm having is that during backtesting, the bot will only ever execute 1 order. This doesn't make any sense, because the Print() that I've put in each of those blocks are printing to the log evidence of a lot of trades. In short, those if blocks are being executed as they should be, but for some reason ExecuteMarketOrder will only every be executed once during backtesting.

The code:

protected override void OnBar()
        {
            if (long_position != null)
            {
                ModifyPosition(long_position, long_ATR_stop(long_position.StopLoss), null);
            }

            if (long_position == null && short_position == null && long_crossover() == true && long_bias() == true && above_avg_vol() == true && ADX_long_ok() == true)
            {
                // open_long_position();

                long volume = (long)(Math.Floor(Account.Balance) * Account.Leverage) / trade_size;

                if (volume > max_trade_size)
                    volume = max_trade_size;

                long_position = ExecuteMarketOrder(TradeType.Buy, Symbol, volume, label, stop_distance, null).Position;
                Print("Long Position Opened");
            }

            if (short_position != null)
            {
                ModifyPosition(short_position, short_ATR_stop(short_position.StopLoss), null);
            }

            if (short_position == null && long_position == null && short_crossover() == true && short_bias() == true && above_avg_vol() == true && ADX_short_ok() == true)
            {
                // open_short_position();

                long volume = (long)(Math.Floor(Account.Balance) * Account.Leverage) / trade_size;

                if (volume > max_trade_size)
                    volume = max_trade_size;

                short_position = ExecuteMarketOrder(TradeType.Sell, Symbol, volume, label, stop_distance, null).Position;
                Print("Short Position Opened");
            }
        }

Unless ExecuteMarketOrder has some conditional operators that I'm not aware of, I can only assume that this is a bug. Why else would all of a block of code apart from one line be executed???