Multi-Timeframe results don't line up

I wrote an indicator that finds the net change between the current price and the open of "Lookback" periods ago. The code is below:

// -------------------------------------------------------------------------------
//
//    NetChange.cs 
//
// -------------------------------------------------------------------------------

using System;
using cAlgo.API;
using cAlgo.API.Internals;
using cAlgo.API.Indicators;

namespace cAlgo.Indicators
{
    [Indicator(IsOverlay = false, TimeZone = TimeZones.UTC)]
    public class NetChange : Indicator
    {
        [Parameter(DefaultValue = 30)]
        public int Lookback { get; set; }

        [Output("Main")]
        public IndicatorDataSeries Result { get; set; }

        public override void Calculate(int index)
        {
            double NetChange = MarketSeries.Close[index] - MarketSeries.Open[MarketSeries.Close.Count - Lookback];
            Result[index] = NetChange * 10000;
        }
    }
}

The code works as needed. But I am trying to make a multi-timeframe version of this but the results greatly differ from the original. The code for the multi-timeframe version is below: 

// -------------------------------------------------------------------------------
//
//    NetChange MTF.cs 
//
// -------------------------------------------------------------------------------

using System;
using cAlgo.API;
using cAlgo.API.Internals;
using cAlgo.API.Indicators;

namespace cAlgo.Indicators
{
    [Indicator(IsOverlay = false, TimeZone = TimeZones.UTC)]
    public class NetChangeMTF : Indicator
    {
        [Parameter("Main Timeframe", DefaultValue = "Hour4")]
        public TimeFrame MainTimeframe { get; set; }

        [Parameter(DefaultValue = 30)]
        public int Lookback { get; set; }

        [Output("Main")]
        public IndicatorDataSeries Result { get; set; }

        private MarketSeries MainSeries;

        protected override void Initialize()
        {
            MainSeries = MarketData.GetSeries(MainTimeframe);
        }

        public override void Calculate(int index)
        {
            int MainIndex = GetIndexByDate(MarketSeries.OpenTime[index]);

            double NetChange = MainSeries.Close[MainIndex] - MainSeries.Open[MainSeries.Close.Count - Lookback];

            Result[index] = NetChange * 10000;
        }

        private int GetIndexByDate(DateTime argDateTime)
        {
            for (int i = MainSeries.Close.Count - 1; i > 0; i--)
            {
                if (argDateTime == MainSeries.OpenTime[i] || (argDateTime > MainSeries.OpenTime[i] && argDateTime < MainSeries.OpenTime[i + 1]))
                    return i;
            }
            return -1;
        }
    }
}

When the NetChange MTF is placed on the same timeframe, it works just as well as the original NetChange.

Any help would be appreciated, thanks!