The most common reason for this is the time frame you selected. Since there is limited spacing for all the orders per price level, the larger your time frame, the less space you will have to print the order flow numbers. We highly recommend you go with smaller time frames such as 5 minutes or less. Depending on the volatility and volume of the instrument you trade, you will need to adjust the time frame accordingly.
The next common issue is if you have other indicators on the chart and they are far away from the current price, you will need to go into the indicator settings and uncheck the "Auto Scale" under the "Visual" section. Once you do this, the indicator will not show if it is too far away from the current price.