This is what is happening . I am at max plunder when i attck a eb at 100% soldiers i get an amount. I bought a new alley my max plunder amount stayed the same but plunder amount i get went down.
As your bfa increases you can be "too strong for an eb" causing a minor plunder reduction. However this generally relates to larger players. Your troops were probably not fully regenerated.
Turn your hat around backwards and put all your change in your left pocket. That should fix the issue.