Det kan være mange grunde...
det bedste er, at du prøver...fanger... undtagelsen og tjekker fejlene ind
from pymongo.errors import BulkWriteError
try:
bulk.execute()
except BulkWriteError as bwe:
print(bwe.details)
#you can also take this component and do more analysis
#werrors = bwe.details['writeErrors']
raise