Splunk search no subsearch - splunk

Question

I have events something like:
{
taskId:5a6d
category:created
when:1517131461
...
}
{
taskId:5a6d
category:started
when:1517131609
...
}
{
taskId:5a6d
category:ended
when:1517134657
...
}
For each task (task id is same), we have events when it is created / started / ended.
I'd like to search if there is any task never be processed (task is created but not started). Here is my search statement:
index=XXX sourcetype=XXX category=created | search NOT [search index=XXX sourcetype=XXX category=started | fields taskId]
This statement works correctly if the time range is less than 48 hours.
If the time range is set to, for example, latest 7 days, the above search statement works incorrectly. It returns a lot of tasks (category=created) which means these tasks are never processed. Actually, they are processed, I can search the events (category=started) by taskId.
I have no idea what's wrong with it. it seems subsearch doesn't return correct results in the range of main search.

Answer 1

This will be hard to debug without seeing your exact data.
To make it simpler, you can try something like this to do everything with one search:
index=XXX sourcetype=XXX category=created
| eventstats values(category) as categories by taskId
| search categories = created NOT categories = started