I know EXACTLY what you are describing.

Not only have I seen it in others, I have often experienced it myself.

And, while I do not know exactly what IS going on, I can confirm that it is a reality and perhaps offer you a few guesses…

In MY experience, it has to do with ‘importance’, or how much something ‘matters’.

Writing about something you like, but which is just ‘personal stuff’ (car problems, games, etc.) can be much easier because it is a ‘light’ subject ;matter.

An ‘assignment question’ is ‘serious stuff’.

It’s like a block pops up and the self-censoring thing – on a completely sub-conscious, not controllable level – puts up a block.

I experience it ALL THE TIME.

Some days, when I want to write a post about really important stuff, I can’t seem to be able to write it up. It took me 10 months(!) to write up a post about ‘reasonable expectations’ of an Aspie student….and even then I didn’t get it ‘right’!

Knowing more and more people rely on my advice about Aspies is pretty much paralyzing me when I try to write a post up about it. What if I steer them wrong? What if I word it poorly and people will not understand? What if i make it worse? What if people read it?

But, when I am answering a comment, I don’t expect a lot of people (except the one who asked it, perhaps one or two others) to be reading it and so it makes it less ‘risky’ to type up the answer – and it flows pretty much effortlessly!

Perhaps it is a weak parallel – but I do think it is a parallel…

Plus, here, I am writing it up as if it were a conversation – not as if I am making a formal presentation. That makes it much easier…

Strategies (which work for me – and are hopefully portable):
1. Talk about it (or, in my case, type it the comments) as if you were having a conversation. Record that conversation. Either by using a recording device, or by typing as you speak, or by having someone else write or type up your words as you speak them.
2. Take a lot of caffeine – it DOES work a little bit like ADD/ADHD meds do in lowering the ‘barriers’. Avoid sugar/sweeteners – they minimize the function of the caffeine. (Yes, this is self-medicating….but, at least, caffeine is not a narcotic or anything…)
3. Put the ‘words you spoke’ into a word-processing file, version 1
4. Re-organize the sections into the proper order for whatever format you require them to be in.
5. Edit the words to form paragraphs in the proper format.

(I sometimes answer the comment, then copy and paste it and re-work it into a post of its own. When helping my kids, I’d let them talk and transcribe what they said, type the words (approximately – I could not transcribe fast enough to get everything) into a word-processing file and let them edit it into the format they needed their answer to be in for school.)

Alternately:
Sometimes, it helps to talk it through back and for – with another person as a sounding board, but in a pinch, with one-self. This helps clarify the importance of the various points, perhaps even the language which is most effective in expressing it, what comes ‘out’ clumsily and needs to be ‘tidied-up’, etc.

As one is speaking, jot down the ‘big’ points as they are being spoken…just as a word or two of a reminder. Then, ‘saying’ the answer out loud, take the set of ‘big points’ as they were jotted down and build from them the answer.

This is much harder and gets worse results, but, it is a method that most teachers will permit in high-school and so on, so it may be a starting point….

Again, each Aspie is different, each one will need to build a set of strategies which capitalize on their individual strength. Yet, being stronger verbally than in writing seems a big part of the pattern for many of us.

Therefore, building a methodology that uses the strong verbal skill, records the ‘spoken word’ in some format, then edits it to fit the required written format might be a set of skills we may indeed be worth the investment!