1) When tracer misbehaves, check the pega log file of the correct node (often there are multiple nodes so be careful!) to see whether any errors show up as clues. It is best to check the entire log file from system start through the point at which you attempted to use the tracer, since other errors might be clues as well.
2) Check your tracer ruleset settings. If you inadvertently have the rulesets turned off, I would imagine that would cause the symptom you describe.
3) Turn on script error reporting in your browser to see whether there are some errors occurring that weren't showing up.
4) Think back to when it started occurring. What had been changed around that time that might offer a clue ?
5) Trace it. No sorry, only kidding, couldn't resist . . .
6) Run fiddler and examine the text-view of the transactions to look for clues. Also, If you have another system on which tracer runs fine, try to make everything as similar as possible, then compare the fiddler results to see where they differ.
Check your tracer settings modify below according to attached image. Problem is with the "Max events to display" You or someone might be changed the value to 0 in your tracer settings "Max events to display" . So tracer never trace any single event.