Hi watkins.trina! Good news: your phone is fine. What you're seeing are Advanced Messaging icons. Think of it like a delivery confirmation/read receipt on an email, but on your text messages! One check means delivered, two means read. Not every message will show these icons because in order for them to appear, the other person has to also have an Advanced Messaging capable phone, and be on T-Mobile Wifi Calling or 4G LTE with VoLTE enabled. As their coverage passes from one band to another, you might see the icon on some messages but not others. That doesn't mean that they're not being delivered, though
Your phone, the Stylo, didn't launch with Advanced Messaging, so you must have received it after the upgrade to Marshmallow! We're rolling this functionality out with software upgrades, so as we make more equipment compatible and the 4G LTE availability increases even more, you should ultimately be able to see these icons any time you're messaging with another TMO user.
Ok awsome. Thank you