I should have also asked how long ago you gave your clam the fw dip. I have seen clams recover from pinch mantle hours after the fw dip, and then again I have seen some clams recover after days. I would give it some time to see if it starts to turn around. It took my crocea a few days to recover after I gave it the dip.
The only reason I gave mine such a long FW dip was because I saw a presentation on clams were many people were giving them FW dips for that long, if not longer. I should also mention that after the long fw dips some of the clams did not make it.
I also had a clam that once got stung and it seemed like it had a pinched mantle. Do you have anything near the clam that may have stung it? (Mine got stung by a fungia. I have since replaced the fungia with another clam.)