The ReCaptcha should not make a difference, it should invalidate the comment form and show errors.
As Balloonatic implies, review your comment settings as a first step.
Because you say this only affects like 5% of your articles, I would check the Options tab on the affected articles, in there is a check box for "Allow Comments", is this unticked on the affected articles? If so, there ya go.
The other things to check are :
- Flood Control (Have you just posted a comment)
- Member Group Comment Posting Priviledges
There is a flood control mechanism in EE, if it's a case of comment section disappearing on articles where I've just posted a comment I'd look into that (pretty long shot).
There are also a bunch of comment settings in the member group priviledges, check these as well. Are you looking at the site as a guest or while logged in?
- Comment Posting Priviledges
- Can submit comments
- Exclude member from comment moderation
- Comment Administration
- Can Moderate Comments
- Can view comments in channel entries authored by others
- Can edit comments in their own channel entries
- Can delete comments in their own channel entries
- Can edit comments in ANY channel entries
- Can delete comments in ANY channel entries
I'v highlighted some of the more applicable ones, have you checked the authors of the entries affected? perhaps this is affecting the 5%?
Edit 1 Some more ideas
Short of jumping into the core code and sticking some debugging trace style messages in there (line 148[ee2.10.1] of file /system/expressionengine/modules/comment/mod.comment.php is a good starting point if your interested) I'd ask you to compare an entry that works 100% and one that doesn't and note down any and all differences in the hunt for clues.
Perhaps try using the entry_id parameter in case there's something fudged in the dynamic handling on exp:comment:entries?
Also whats the status setup and expired/future entries setup is on this channel? Theres code in the comments module that filters comments on statuses (I've seen issues with this too!) and expired/future entries.
