A possible cause for this is a very large signal which exceeds the spike limit value. The spike limit should suppress spikes in the spectrum which can occur from time to time and which are usually limited to one channel. Thus, if the intensity of a channel exceeds the spike limit value, the current scan is discarded. However, if the signal is very strong and exceeds the spike limit value, all scans exceed the limit and will be discarded resulting in the mentioned behavior. To solve this, increase the spike limit value in the scan parameter window.