One thing not yet mentioned but most likely not related, is that the external levers often will not stay tight enough to prevent the levers from slipping just a little on the flats; and so, it will not hold it's Neutral gate adjustment. This can be a hard frustrating thing to find.
But as to your issue, Here's another possibility;
When you press the clutch pedal down, And the driveshaft stops spinning, AND the clutch disc ALSO stops spinning as per the design; it can happen that, as you lift off the pedal, the brass locks on to the brake. With both the clutch teeth on the chosen gear, and the slider teeth, NOT lined up enough the slider will not/can not affect a shift. When this happens it will not go into gear.
The solution is to cause one of those two to move, just a tiny amount.
That usually means to put the trans back into neutral, and lifting off the clutch, which breaks the brass off the gear. Once free, you can start over, but this time, you engage while the gears are still spinning just a little. It doesn't hurt the trans, it just takes getting used to.
The other way to handle this, is to select First gear while the driveshaft is still spinning, even just a little; which hopefully, will prevent the brass from locking on. This is usually done by leaving it in Second as you come to a stop, dragging the idle down a bit, then, just before the car comes to a stop, slam it into First. Depending on your gear ratios, you can fine tune this to a specific roadspeed and rpm at which the stick will freely go into and come out of First gear, without using the clutch.
Like say you have 3.55s and the idle speed in Neutral is 750, and you have 27" tires.. the perfect downshift-speed into first, will be about 6.4 mph. On mine, my rpm is 700, and low gear is 3.09, so the perfect downshift no clutch required speed is 5.1mph.
There are only a few possible cures that I know of for this situation.
1) if you are running synthetic, I don't care what your brother in law says, get rid of it, and flush the trans out. That synthetic will stay in the hollow cluster reservoir, and it will pollute whatever oil you put in there, and that Synthetic coats everything, and is hard very hard, to flush out. In my case I had to drop the trans and actually take it apart to physically wash it out.
2) If you already have a thin oil in there, replace it with a thicker one that prevents the brass from locking on TOO quickly, and
3) is to pull the trans down and fix the brake-cones and points, so it is less likely to lock in the offset position.
4) First gear requies a special treatment.
When I rebuild an A833, I chuck each gear onto a lathe and deglaze the brake cones. This just takes a couple of swipes with 120grit emery cloth on a big flat file. If you look at the cones on higher mileage units, you may find at the base of the cone, an unworn step. You gotta take that off. And what I do is bias the strokes to put an invisible screw onto the cones ........ in the upshift direction. This is to help slow the gear down faster for a quicker upshift. Yes it really does work.
However, when it comes to reverse, this a wrong thing to do, as there you are at a stop-sign with the brass screwed on tight, and I see you hammering on the stick again. Yeah don't do that.
So on First-gear, I bias the deglazing on the downshift side, or not at all, depending on the customer. For the casual car-show attendee, he gets NO bias, and a good straight finer polish, to help that gear NOT lock on. For the hotrod streeter who wants to pretend he's the fastest downshifter on the face of the Earth, but hasn't mastered blipping the throttle at the right speed and time, an aggressive bias on the downshift side, will impress him with my work, lol. But the penalty is a slower N>1 gear selection, so then clutch departure becomes more important, and; a tiny amount of drag between the disc and the PP is gonna help that gear to NOT stop with the clutch teeth offset too far to be able to bulldoze over.
Yeah so, that's been my experience. I hope I explained it well enough to understand.
But as others have said, for most of us, you gotta get the disk to stop dragging; and this does NOT mean with the clutch pedal to the floor. I set my freeplay pretty tight, and adjust my seat so that I can just get the pedal to where it needs to be..
The penalty is more frequent adjustments.
The benefit is that, with the other changes I made, my trans shifts like lightning, even at 7200.........
On that note is this;
The slanty pedal ratio is smaller than the V-8s, so then, if you have a slanty pedal in your car, you will actually have to push the clutch pedal down waaaay further for a given departure. Yes it can be modified and the mod has been published right here on FABO.
But if you have to pay someone to do it, it'll likely cost more than just swapping in the right one. AND
if you have a hi-perf 3-finger B&B clutch, you'll need the HD pedal braces anyway.