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I am building a table but I'm not sure why the following is happening

The fields are

First Name
Sur Name
Day
Date
Project Name
Project Code
Hours

all but hours are look up tables

I put in say 100 enters

There are 9 entres which have Thursday 13th and 9 entres which have Friday 14th

I save, come out and go back into the table to find these entres have moved from their position and now sit in a group of entres for someone else or they are at the bottom of the table?

What can I do to prevent this?

Thanks