Updating FIFO chart on blocks diminishing the FIFO capacity

I am working on a multithreaded application where the acquisition and chart display run on different threads. I am attempting to collect samples and plot only when I have 100 samples available to have less resource consumption and keep the application responsive. However, when I change the number of samples in the block, my FIFO capacity seems to change, allowing significantly less amount of samples than the ones I need. The current FIFO capacity should allow for at least 16 mins worth of data, but it only shows less than a second

If I set the block size to 1 (single sample appending) I obtain the results I need, but I am seeing performance issues in other areas of the program, hence the need of appending in blocks.

See the attachments for more clarity. Any suggestions?

EDIT: Adding code

private void DisplayNPD()
        {
            XyDataSeries<float, float> npdRawDataSeries = new XyDataSeries<float, float>();
            int fifoSize = 1000000;
            npdRawDataSeries.FifoCapacity = fifoSize;
            npdRawData_RS.DataSeries = npdRawDataSeries;

            double npdRaw = 0;
            bool successfulDequeue = false;
            int samplesQueued = 0;
            int samplesInBlock = 100;
            float[] rawSamples = new float[samplesInBlock];
            float[] time = new float[samplesInBlock];

            while (!ImagingSession.terminateThreads)
            {
                if (ImagingSession.laserOnOff && !graphRestarted)
                {
                    int npdElementsInQueue = npdDisplayQueue.Count;
                    if (npdElementsInQueue > 0)
                        successfulDequeue = npdDisplayQueue.TryDequeue(out npdRaw);

                    if (successfulDequeue)
                    {
                        currentTime = graphStopwatch.ElapsedMilliseconds * 0.001;
                        time[samplesQueued] = (float) currentTime;
                        rawSamples[samplesQueued] = (float) (npdRaw * 1000);

                        samplesQueued++;
                        if (samplesQueued == samplesInBlock)
                        {
                            using (npdRawDataSeries.SuspendUpdates())
                                npdRawDataSeries.Append(time, rawSamples);
                            samplesQueued = 0;

                            if (currentTime > upperLimit)
                            {
                                lowerLimit = upperLimit;
                                upperLimit += xAxisWidth;
                                AdjustXAxis(currentHorizontalScale);
                            }
                        }
                    }
                }
            }
        }