People have failed and not been given the opportunity to resit, and others have failed and been offered another go after some re-training.
Much the same as every other airline in the world - people don't normally get one go and thats it unless there is a reason.
If you are genuinely trying your best and just fail on the day in all likelyhood you will get another go. If you go into the sim not knowing your stuff, and have obviously not been putting in the effort you may not..it all depends on the day.