Allen Wrenches Don't Fit In Truss Rod?

long shot but maybe it needs a metric key?

 

If the wrench that came with it doesn't fit,. Be careful how you proceed. Getting the right angle of attack may be difficult, and you absolutely don't want the wrench to be only partly seated into the recess.

Try a flashlight and a probe to see if something is blocking the recess in the head. If not, try again. It's worth having a set of SAE and Metric hex wrenches around to assure you have one that fits.

 

I had an MIM i couldn’t get a wrench in and a closer look revealed there was a gob of glue in the socket.
I dug at it for a bit with a straight dental pick and that did the trick
Also, all my MIM basses are/were used so i never got a wrench with any of them. I later got a couple of Highway One (1) basses that included the official Fender ball end wrench and a heel adjustment but the wrench worked in my MIM head adjust truss rods.
I keep a set of metric and standard Bondhus ball end L wrenches in my bass tool kit but i mostly use the Fender wrench. I occasionally get ropped into doing something on anther brand of bass so the L set(s) have been used, and i replace any slotted saddle screws with socket heads and have a couple of Babicz bridges so theres that, but the bulk of my truss rod adjustment work is with the official ball end T wrench.

 

I adjust MIM with the same 3/16" T wrench used for the MIA basses as they use the same nut.

 

If it ain't standard it's metric.
If it ain't metric.......

 

If you have a glob of stuff in the slot I would be careful about what may be on the threads. It may be a good idea to take the nut off and clean the threads a put a tiny amount of lubricant on them.

 

So I will say this about MIM truss nuts...

I've worked on a LOT of the MIM basses over the years. And one thing I do when I overhaul one is to remove the truss nut and clean it thoroughly before reinstalling it with grease on the threads and possibly a washer underneath.

If you can find the proper wrench (and it should be 3/16", but could be 5mm), you should unscrew it completely, take it out of the neck, use steel wool and/or a wire brush to remove all of the stuff that is usually attached to these (glue, polishing compound, wood, rust), then make sure the wrench hole is also clean. When reinstalling, dab some grease on the threads. Silicone grease is great, but anti-seize, lithium or any other thick grease will do.
Hell, any lubricant is better than the none that Fender uses.

Why do this? It will allow the truss rod to function without the binding that is associated with a filthy, ungreased nut.
Will the truss come loose like that? No. I've done this a lot and it's never failed.

 

A cutoff from an E string with a little hook bent on the end works nicely for cleaning gunk out of the allen head socket.
I would avoid using ball end wrenches if you can help it; the ball end applies a lot of torque over a very small surface of the nut and can lead towards stripping. A regular allen wrench is better in this respect.

 

Not much else to say, after this...

 

This...

 

Maybe it will add some sustain, or fix a dead spot?