Not generating a dump happens for one of a few common reasons:
1. A process crashes (lsass or csrss) that controls access to the security subsystem, meaning permissions checking fails, and crasdump cannot write due to lacking permissions to write to the paging file (where memory sump data is written).
2. The system memory committed at time of a system crash is larger than the paging file's size when the crash occurs, resulting in a hung or failed dump write to disk.
3. A driver or process that is "boot critical" or hooks kernel APIs crashes or hangs during the bugcheck, thus causing a crash of the crash, and no dump is written.
There could be others, but those are the most common I've run into over the last 15 years or so, and in that order of most common to less common.