I can’t figure out why some words are blocked. According to the order of the lessons, I should have learned them quite a long time ago. And I am already familiar with all those characters.
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Welcome to the forum!
Our word selection algorithm considers a few things, which may delay some of the words.
We account for things like:
- where it is in the lesson order (derived from character items), but also
- how many words you already learned for each character of the word
- the overall words you’ve learned from the characters of the word
- the HSK positioning
These checks ensure that you don’t learn a character and then not learn any words that include the character – ultimately to help reinforce the character 
I think this raises a good point that the “Lesson Order” for word items can be misleading and confusing though 
If you want to learn those words, you can prioritize them to round out those delayed words in the meantime 
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Most of those words are also recently added some days ago which may be why they haven’t shown up quite yet. As Phil mentioned, the word selection algorithm will try to focus on “broad character coverage” (i.e., getting an even character-distribution in the words selected), which is why this may not have shown in your queue quite yet.
As we start adding more and more lower-level HSK words, we are also looking into potentially adding an option for users to change the word-selection algorithm to prioritize words based more on HSK level than “character coverage”.
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I think this is the case for all of them cause they are also grayed out for me. I’m at a studied words count of around 1000 at the moment. It must be because they’re new words.