I am running a recorded Test Plan in which, I have assigned 5 different users to the Thread Group, with a Ramp-up period of 5 and Loop Count of 1.

This is what happens after executing the script:

The test runs without errors and when I verify the results in 'View Results Tree' listener, I find all users got successfully executed without any error, but when I verify the same in system; only 2 or 3 users got registered. I deleted the registered users and ran the script again, and this time too only 2 or 3 users got registered (this time different 3 users out of 5 got registered) i.e. user selection for registration is random, but count remains only 2 or 3 out of 5 users.

I don't understand 

1. What can be the possible reason for this behavior of my test?
2. Why my Test Plan is not registering all the 5 users?

I've checked the resource utilization on my machine too, but they were fine, no problems with that.