I faced similar issue around 5-6 months back, and the main reason for that was the tests in that suite were getting timed out. This time-out is there in the Advanced Settings section. By default it is 180 mins. Meaning if the tests in the suite are not completed in 180 mins then that suite gets cancelled.
Try setting it to max value i.e 999 mins.
At least this worked for me at that time.
