Phillipa,
In your Logs report, check for the JMeter or BZT log, and look for the total number of Threads started.
Should look similar to i.e.:
Thread group started 1-1,
Thread group started 1-2,
Thread group started 1-3
......
Thread group started 1-50
If that's the case, it will mean that ALL thread were started, and finished within the duration of your test, and just did not run all 50 threads concurrently.
If you need more details you can reach your to Blazemeter Support
Regards
Javier