This is not the first time I've had this issue, but it's infrequent so I tend to forget.
To clarify, on occasion when posting an item it will not appear when searching the entire AH for it by name - although the exact conditions tend to vary. However it will turn up by searching the category and using the appropriate rarity threshold (common, epic, etc...). This is usually how it turns out, although as I mentioned before it varies.
Giving a specific example, right now on my character Garik Shadowgait I have a Frostburn Enchantment selling for 600k (while stocks last!). If you search the Entire AH for "Frostburn Enchantment", specifying Epic to avoid shards showing up, it will list every lesser, regular, greater and perfect Frostburn enchantment on the market EXCEPT mine.
Trying the same search again, this time limiting to Refinement>Enchantments, and it will appear - as will everyone else's.
On previous occasions though, it would generally only work as I specified earlier (won't turn up searching by name, but will using categories and search modifiers).
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