If you are noticing issues with messages not syncing to MightyText while using Android 6.0 (Marshmallow), please do the following:
1. On your Android phone, launch the Settings pane (main Android settings)
2. Select Battery.
3. Click the Overflow settings (triple dot) button.
4. Click Battery Optimization.
5. Click Not Optimized and choose to display All Apps.
6. Scroll down and tap MightyText in this list.
7. Choose Don’t Optimize for MightyText.
8. Tap DONE.
Once this is done, give it another try and let us know if you continue to see the same issue.