Thanks for reaching out, let's 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 Chomp SMS 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:

Samsung S7 phones: Android Settings > Battery > MORE (top right) > says 'Optimize battery usage', locate Chomp SMS and turn OFF.
Other Samsung Phones:  Android Settings > Battery > App power saving [DETAILS], locate Chomp SMS = Off / Disabled
Other Phones:  Android Settings > Battery > Settings / Cog or dots icon (top right) > Battery Optimization > All apps > Chomp SMS = Don't Optimize  

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

4) Elephone or Blackview: Set Android Settings > Background task clear = Off (or add Chomp SMS 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.