As an instructor I get to ride quite a few different small motorcycles. I know that difficulty in finding neutral generally has a couple of components.
First, it does take a delicate touch on the shift lever even if your bike is in good "normal" condition. A heavy foot will blow right through neutral, even when it is not that hard to find. Often a student will get frustrated trying to get neutral and one of the instructors will check it out. Most of the time its not a problem and the student just needs a little technique refinement.
Occasionally however, even an experienced instructor can't get it into neutral, and the bike is clearly experiencing a dragging clutch. This is when pulling up or pushing down on the shifter with clutch pulled meets a lot of resistance, and then finally the pedal lets go and it is virtually impossible to stop the motion in the middle at the neutral detent and it blows right through into first or second. This is a result of the undercut dogs on the gears as Kryton noted. With the dragging clutch, the input shaft has some static torque on it which creates resistance to pulling the dogs sideways out of the slots on adjacent gears because of this undercutting. The undercut dogs are also why you cannot shift gears while the engine is accelerating under power. You can apply quite a bit of pressure to the shifter lever, but it will not move until you roll off the throttle to unload those dogs. This is a design feature of ALL motorcycle transmissions. The dogs tend to drive themselves into the slots under power to prevent any tendency to jump out of gear.
Our training site had the misfortune to buy several Honda Groms a couple of years ago. This batch of Groms were ALL afflicted with dragging clutches, and there was no way to get them into neutral with the engine running. All of these bikes went back to the dealer for new or repaired clutches, so faulty clutches can be present right out of the box. A few years back we had a similar issue with Suzuki DR250's and the dealer was not able to repair them. As a result, we have not bought any more Suzukis.
So, there is no big mystery here. Some Rebels (like mine) will slip easily into neutral, hot or cold when the engine is running, Others have a degree of difficulty doing so, and others will not, under any condition, do so. Unless there is an issue with the shifting forks or linkages, (rare) the problem is due to failure to fully disengage the clutch resulting in some static torque on the input shaft, resulting in high friction on the undercut shifting dogs on the transmission gears, resulting in high force being required to move the shifter lever, resulting in inability to find neutral while the engine is running. It's really that simple.
2004 Rebel 250, 2003 BMW K1200GT (roadburner), 2004 BMW R1200GS(all purpose),
1973 Norton Interstate (in a box in the basement)
1968 Triumph Bonneville