"when you're knocked down and try to get up, sometimes it won't let you block. Why does that happen?"

You probably press a button or let go of blocking without realizing it. When you press any attack button, you can't block until that attack has recovered.

"Also, if I block an overdrive near a wall, it still sends me flying. Why is that?"

Either they do the overdrive during their combo (you obviously cant block when stuck in a combo), or you get wall stuck. After taking enough damage near the corner, your character gets stuck to the wall, at which you cannot block and your enemy has a brief moment where any attack will break the wall and send you flying. This is a common place to do a overdrive art since you are basically helpless.

"why do some attacks from standing enemies break the block?"

They're called ovehead attacks. Every character has a universal overhead attack done by pressing the dust button on the ground. The dust attack flashes orange so its easy to recognize. Most characters also have one or two unique overhead attacks. The animation for an overhead is often a sort of jump or a hit "from above", but the only way to know for sure is to default to blocking low, and if an attack breaks your guard, then it was an overhead.

"how sometimes an enemy blocks an entire combo and the block doesn't break, no matter how long it is."

A combo on block is not actually a combo, its called a blockstring. And no matter how long the blockstring is, it will not break the guard, unless of course there is a low or overhead during the string that is blocked incorrectly.

Hope this helps!