Receive delays / sending delays / not receiving messages / messages late / scheduled messages not sending (Xiaomi HTC Samsung Asus Elephone Blackview Huawei) / delayed / battery

Thanks for reaching out, lets get this sorted out.

1) The cause of your issue is most likely Cleanup, Battery, Speed Improvement type apps, for example Cleanmaster, CM Suite, CCleaner, Battery Doctor, Greenify, Amplify and apps of this type.

You need to add Textra to the excluded list inside the culprit apps settings (if available).
 
2) Also some phones stock battery saving preferences can cause an issue, check as follows:

Huawei: Android Settings > Battery > 'Close apps after screen lock', turn OFF the entry for Textra. 
Huawei Mate: Android Settings > Apps & Notifications > Textra > (Advanced) Battery usage details > Run in background = ON
Huawei Mate: Open the App "Phone Manager", then in Settings > Smart tune-up turn it OFF.
Samsung S7 phones: Android Settings > Battery > MORE (top right) > says 'Optimize battery usage', locate Textra and turn OFF.
HTC 10: Android Settings > Power > Battery optimization > Textra > Don't optimize
Xiaomi: Android Settings > Battery & Performance > Manage apps battery usage > Choose apps > Select Textra > In background settings set to No restrictions. 
Pixels: Android Settings > Apps & Notifications > Textra > (Advanced) Battery > Background Restriction = 'Background usage can't be restricted'
Pixels:  Android Settings > Apps & Notifications > Textra > (Advanced) Battery > Battery optimization = 'Not optimized'
Other Samsung Phones:  Android Settings > Battery > App power saving [DETAILS], locate Textra = Off / Disabled
Other Phones:  Android Settings > Battery > Settings / Cog or dots icon (top right) > Battery Optimization > All apps > Textra = Don't Optimize  

3) Asus phones: Android Settings > Auto Start Manager [DOWNLOADED], set Textra to Allow.

4) Elephone or Blackview: Android Settings > Background task clear = Off (or add Textra to the whitelist)

Important: Please check carefully and report back, this will help us sort out your issue and help us to help others too.

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