You're assuming that the castle nut indexes to the exact spot it was originally. Unlikely. There's also more than one spot to stake on a castle nut.
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@ Ryno12: Do I need to stake all 3 slots on the castle nut? And, are you saying NO RED LOCKTITE or NO LOCKTITE AT ALL?
Stake in two places but NO LOCTITE ! Ever !
I didn't realize it was kosher to reuse the same endplate and castle nut. Didn't realize you could break the stakes with a wrench either; figured it would be too much torque. Good to know!
I stake 2 and 40 ft. lbs. of torque on the receiver extension nut will do ya.
To the OP, lots of info:
https://www.m4carbine.net/showthread.php?t=91530
Break it free with a proper wrench, reinstall with new endplate and nut. Two of the three notches will be available to easily stake. I have had success using a straight bladed screwdriver and tilting it to one corner in order to move material from the endplate into the castle nut notch.
98% of the time you can only stake 2 of the 3 positions. This is normal and is all that is ever needed.
Only once have I seen a castle nut line up perfectly (by luck) where you can stake all 3....one slot has to be exactly at 12 o'clock.