As per the attached screenshot the connected systems (collectors | agents | clients) were 0 from 15:40 to 15:50 secondly PurePaths status Y-Axis is too high from 15:05 to 16:00 (0-20000), therefore, in between PurePaths you can not see.
Notice that the scale on the PurePath status chart is nearly 80,000 due to the large bar in the timeframe, whereas the other charts for PurePaths are in the hundreds. I bet if you select a timeframe without the high values or manually lower the axis you'll be able to see values in that chart. The value is just too low to see it given the scale.
Also note that the transaction flow is based off of PurePaths in the session so if you have a transaction flow you must have PurePaths at least at some point within the timeframe you're looking at.
I have selected the 5min, 15min interval as well then also we can see any purepaths during that time. But we have request in web servers.
Also we have intermittent application service down issue.
Did you correlate the timing of missing PurePaths while Dynatrace Server was not available?
What is the size of session storage?
Do you have a local storage for the sessions data or the NAS/SAN?
Did you review the Dynatrace server logs?
Did you review the events of the OS for the same?
Would you mind to share the Dynatrace server logs with us.