The first thing to know is that the NN interval data are filtered data. In fact, the calculation of the NN interval is based on the RR interval (for more details, please refer to API documentation). Therefore, it is possible that there is a gap in the data because unreliable RR intervals were excluded from the calculation. If you want to detect all the QRS complexes without filtering, you might want to use the RR interval data instead of the NN intervals.
Please note that to capture consistent data (i.e. avoid unreliable RR intervals), the Shirt needs to fit properly on each participant (for more information, refer to this link) and you need to make sure that all the sensors are in contact with the skin. To obtain a better signal, it is very important to apply conductive gel (or simply glycerin-based cream). Also, fastening the elastic belts will help to keep the Shirt in place and allow for better contact between the sensors and the skin during the recording.