IF: You use Signal as default SMS application and super-optimise your device with Battery Doctor. And you’ve included Signal in the list of exclusions to super-optimise in Battery Doctor.
THEN: You’ll only receive normal sms or signal messages if you have 3g connected. If you have WiFi connected you need to open the application signal to get the new messages.
You can avoid this issue if Signal isn’t set as default SMS application. In that case signal always receive signal messages immediately if WiFi is connected.