I'm not working on X because when I start to look at it my brain seizes up with a formless sense of dread at even the familiar parts of the task and I can't find the "start doing" lever.
I'm not working on X because the ticket for it was put in by that guy and I don't want to deal with the inevitable nitpicking follow-up questions and unstated additional work.
I'm not working on X because if I start doing the easy parts that would commit me to also doing the hard parts. Maybe if I leave it, some other sucker will take it on and I won't have to do it at all.
I'm not working on X because to even get started I would have to figure out how to disambiguate the requirements, and that requires a flexible mode of thought that is a bit beyond me right now.
I'm not working on X 'coz I don't wanna and no-one can make me. X sounds tedious and unrewarding, and there's so much of the internet I haven't read yet.
I'm not working on X because no-one will notice or care that I didn't specifically do X. If anyone asks I can say I was doing Y and Z today, act like they took up more time than they actually did, have an X-shaped amount of extra slack in my day, and get paid the same salary either way.
I am not working on X because it's so poorly defined that I dread needing to sort it out.
I not working on X because I am at a loss where to start
I feel like admiring the problem X and considering all the ways I could theoretically start solving it, so I am not actually doing something to solve it.
These are all sub-types of procrastination. In my experience, thinking about this as "procrastination" is less helpful than ignoring that word entirely and finding the specific reason why I'm procrastinating instead. I'm not trying to redefine procrastination, only saying that you may want to taboo it.
"I'm not working on X, because daydreaming about X gives me instant gratification (and rewards of actually working on X are far away)"
"I'm not working on X, because I don't have a strict deadline, so what harm is in working on it tomorrow, and relax now instead?"
There are specific reasons for why you are procrastinating. There are specific solutions to these reasons. Bundling it all up into the "procrastination" black box, or offering solutions to the "procrastination" problem, is a problem. Specific diagnosis is not always straightforward, but it'll get you farther than "procrastination".
"I'm not working on X because I feel oppressed by a 'should' injunction."
"I'm not working on X because my hunter-gatherer instincts always attribute more weight to the immediate choice ahead of me that implies the least energy expenditure, and I'll be damned if I'm ever calling that 'laziness'."
"I'm not working on X because that would be the very first building block of X, and going from 0 to 1 is the hardest thing ever."
"I'm not working on X because I am dehydrated/sleepy/hungry and that is actually having a noticeable effect and sapping my morale."
"I'm not working on X because X is a big leap and that scares me. The status quo is nice."
"I'm not working on X because I'm working on Y, Z, W, and V all at once, and those are short term and X is long term, and so even though X is more important overall, my algorithm gives more weight to the other letters."
"I'm not working on X because I feel defeatist right now. My P(doom) is one hell of an ugh field."
"I'm not working on Y because Y is my hobby, and I would feel guilty if I did it instead of X. But then I'm not doing X either, so I'm doing the completely pointless thing in the middle where all I do is scroll."
"I'm not working on X because everybody told me working on X was useless or socially embarrassing. I carry my social brain everywhere I go, even when I'm working alone. (2 million years of human evolution, baby.)"
"I'm not working on X because X really, really, doesn't look attractive right now. I know that if I walk back into X it'll be in the same mess I left it in last time, and I'll have to mop it all up."
If you can think of other specific reasons for why I'm not working on X, I'd love to n+1.