If you're looking for them to eliminate compressability, you're going to be unhappy. They merely raise the speed of the onset of compressability, mitigate to some degree the effects, and aid in recovery.
They work exactly as they should, they even give a momentary pitch up when deployed outside of their normal operational envelope.
By the way, you should not reduce power.
If you're locking up a P-38 in a dive, you're doing it wrong.