If you recently received the Verizon or AT&T 5.1.1 Firmware Upgrade, it breaks vibrations in a number of apps, including Textra SMS. Certain system type apps still vibrate, so it appears to impact third party apps only. The issue only occurs when in Power Saving Mode.

The only known workaround is to turn off Power Saving Mode (Android Settings > Battery). This impacts lot's of apps, not just Textra SMS.