Why some of my text messages vanishing?

Hello everyone here!

I have just recently upgrade from Classic to Passport. I love the phone and I am very happy with it. What concerns me the most is randomly loosing some text messages. Complete thread have gone missing and I am not able to get it back. I am using the phone just for a business purposes and every single conversation with a customer is important and loosing it is absolutely no acceptable. Would you be please able to sort it out this issue? If it can be fix.......

Thank you for any comment

Radim