If it's an alloy rack, check that the bolts on the bottom aren't hitting any of the web strengtheners on the top of part C5, with some setups and screw lengths, a little grinding may be needed.
Other issues I've had, have been the screws working loose at the top and bottom of the alloy tube (threadlock seems to have fixed that), random rocks getting trapped and the servo moving on it's plastic mounts (alloy sorted that). With the stock setup I had issues with sand/grit in the tube on the chassis that the steering runs through - hasn't been an issue with alloy, but if you've been running somewhere very wet or muddy might be worth a check.
I also had 1 alloy setup that just plain didn't work, all the tolerances were out so it got replaced.