It has happened to me before.
Just delete the old crash logs and let the crash happen and you can easily find the crash log.
I would suggest you to use the following option to check for the crash logs in iOS device itself as its easy and quick to do.(depending on the OS version the below selection process will change)
Settings -> Privacy -> Diagnostic & Usage -> Diagnostic and Usage Data
Just connect the iOS device to a machine and sync it with iTunes then all the crash logs that you saw in Diagnostic and usage Data will not be there any more and it'll be in the machine.
Next time when the crash happens you can see the crash log
- under Diagnostic and Usage Data
you can click on the any particular crash log and share it via mail
- you can sync it with iTunes and find it in OSx at ~/Library/Logs/CrashReporter/MobileDevice/
- you can go to Xcode->Devices and Simulators -> View Device Logs