Hello dear,
If you are not getting rid of stains, get the retainer cleaning tablets by the name Retainer Bright 96 tablets or Efferdent. Take lukewarm water and dissolve the tablet in it and follow the timing instructions. If you have tried that even, then the best solution is to go to your dentist, we have cleaning solutions and polishing cups which would do more help.
Next Steps
After that, whenever you eat anything, clean your retainers after that in lukewarm water.