If you recently received the Verizon or AT&T 5.1.1 Firmware Upgrade, it breaks vibrations in a number of apps, including Textra. 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 (General Settings > Battery). This impacts lots of apps, not just Textra.