I have an Odium (Bajarl/Joker) who won't learn mystic combo. I have used mystic punch far more than 50 times and my nature is "good". I was able to learn mystic punch with no problems at this nature (presumably a very low sub-level of "good", which I will explain later), yet mystic combo seems impossible. Is this related to nature? Or is this a more complex tech chain?

This wouldn't be so bad, only I can't improve my monster's nature! It is stuck at this low sub-level of Good. Feeding it a sour jelly actually bumps it back down to neutral for reasons I do not understand (sweet jelly does the same thing). I notice that it was mentioned that jellies do not directly affect nature, but I've never heard of sours bringing a monster lower on the spectrum.

Even with 4 or 5 praises on greats, my Odium is stuck at this level where a single jelly, be it sweet or sour, bumps it back down to neutral.

Does anyone have any solutions?

I think that it's to do with the sub-type joker. In the list of items and their effects Dark Phoenix
wrote "It's also interesting to note that some breeds can only get so good or so bad. In other words, if
you're trying to make your Joker into a saint, forget it:)". So that may be why you can't get higher than
good, I've been experiencing the same problem with my Wracky/Metalner. Does Errow say that you can get
the move and you're just not learning it, or does he say that it's difficult still? If it's the first,
this problem appears to have just happened to both me and my brother over four separate save files, and
you'll basically have to wait until about Stage 8 onwards of life until you can actually get the move. If
he says it's still difficult then I'd say just keep using mystic punch as 50 seems to take a lot longer
than you'd think it would, and just keep praising greats/scolding cheats etc.

"But it might not be so easy to acquire a new technique right now." is what Errow tells me. I made sure to count the exact number of times I used Mystic Punch up until 50, I'm sure I'm at 75 by now. Also, noticing that the technique list named the tech "1-2 Mystic Combo" I figured I may as well use 1-2 Smash 50 times as well in the event that it's one of those more nebulous tech chains (Kung Fu Kick, for example). No luck in either.

It's unfortunate if my Jaba won't go above barely being Good and that such a disposition prevents him from learning Mystic Combo, but the most alarming thing is how Sour Jellies are lowering my nature. Any idea on that one? Thanks a lot for your help.

Looking at my brother's Selketo, he's experiencing a similar problem I've managed to escape. Both the pure
ArrowHead and the Mustard Arrow before it (both combined as the first monster in combination) both could and did
learn energy shots, and this monster also could at the start of its life, but as it's got older it's lot the
ability to learn it. I am totally clueless.

The tech chain list is far from perfect. Parts of it have been edited over the years when we had better data, but, admittedly, there may be parts based on something someone posted years ago without really knowing anything.

Also, there are monsters who cannot ever get all the way up or down the good/bad spectrum. Some types just have limited ranges of nature. I have learned this through direct experience.

Strangely enough, I had a Wracky (Charles, 1st gen) who could only get to good, but after combining it with a draco doll the resulting Wracky was able to get to best. Also, in terms of jellies directly affecting nature, I have experienced on a few different occasions feeding my monsters jellies and it altering their nature, i.e. turning it from neutral to good. So I believe they do directly affect nature.

Absolutely none of what I've read could possibly explain why a sour jelly would cause the nature of a monster to head in the wrong direction.

@Luke
I wonder if min/max nature for monsters could be averaged in the same way that lifespans are when combining?

Also, jellies were touched upon in the "Detailed Guide to Nature (Good/Bad) & Changing Nature by Strunks and Lisa"

It seems they alter a value up or down by 33.

I suspect as with stress/fatigue/others there is a value system from 0 to 100 with regards to nature and giving jellies increases/decreases this by 33, though as I haven't seen any figures on how praises/scoldings affect nature this is just speculation.

Certainly you can't just give a Worst nature monster 4 sour Jellies and turn it to Best (and vice/versa)

It's possible each level of nature is made up of 100 of these nature points, or perhaps even each step within each level. This could explain jellies affecting nature in the subtle way they seem to.