There are a few factors that could affect the level of attention we've collected for a publication.
When was the article published?
Altmetric only collects data about articles from supported publishers / repositories. Altmetric started collecting content from most publishers during the second half of 2011.
Was the article published before 2012? If so, it might be that Altmetric simply didn't support the journal it came from until more recently.
Where was the article popular?
Was it popular with bloggers? We keep a manually curated list of more than 9,000 blogs that we track for article mentions. We add to it frequently but usually can't import more than a few weeks worth of older posts - so it could be that we just didn't track any of the relevant blogs until a little time after the paper was published.
What were the relevant posts linking to?
Altmetric only tracks direct attention: that is to say links to the actual paper. Often news stories will refer to a paper without providing sufficient evidence for us to link it to the appropriate paper. e.g. "a paper by Myers' lab published in this week's Nature" Altmetric needs either a direct link to the publication or for the news article to meet our text mining criteria. There is more information here about how Altmetric uses text mining to collect news stories.
Sometimes news stories will link to an associated editorial, perspective or research highlight instead of directly to the article. Altmetric won't pick this up (the editorial will be scored highly rather instead of the actual research).
Has the attention all been in the past 24h?
Altmetric captures attention in as close to real time as possible but it can sometimes take up to a day for posts to filter through to the bookmarklet or Explorer.
It was published recently and I can see links to it that haven't been picked up!
If we've missed a mention to your publication please do let us know! You can submit your missed mentions using this form and a member of our team will process them as soon as possible.