1. Maybe it's the tailcap retaining ring coming loose and not making contact sometimes. Try tightening it.
2. Maybe it's the Olight IMR ORB-163C05 battery, try a different/new battery, however, if the problem persist, go to (1) - Tailcap issue.
3. If you use the Turbo mode often, make sure whatever battery (including Olight's own IMR battery) is capable of supplying the required power, else, the unit will not switch on.
4. If none of the above solve your problem, contact your reseller to get a warranty replacement since it comes with 5 years warranty period.
I'm not sure whether yours is S1R or S1R Turbo S, the battery in my S1R had an early retirement, while the battery from S1R Turbo S seems more resilient. I also had 2 tailcaps needing replacement. :shrug: