This is usually a character encoding problem. If you’re copying your text from a text editor like MS Word, it will often paste something called “curly quotes” or “curly apostrophes” that have special html needed to display them. My guess is that WordPress excerpts don’t allow for html and that’s why you’re seeing the question marks.
You could try using a plugin like Advanced Excerpt: http://wordpress.org/extend/plugins/advanced-excerpt/ – however, this is not an endorsement as I’ve never used it and it hasn’t been proven to work with the newest version of WP.
Any tips or tricks? The question marks are only showing in the excerpts, but not when you click through to the blog post
Other than the plugin that I mentioned above, there are a couple things that you could do:
1. You could try using manual excerpts if your theme supports them. I don’t believe that they strip the html the same way. http://codex.wordpress.org/Excerpt
2. You could paste your post content in plain text, however this will remove any html/links that you may want to keep from the source that you’re pasting from. This, in my opinion is the best suggestion so that you make sure that you’re not including html that you don’t want in your posts.
3. You can edit your posts and remove any html markup in the first few lines of text that you paste in. So for example, in this post: http://blog.mixrank.com/forging-ahead-with-multichannel-competitive-i/ – You’d go into your WordPress dashboard to edit your post in the “Text” tab rather than the “Visual” tab. Then you would need to manually search for html like ’ (which is what is causing the question marks) and replace it with a normal apostrophe.
4. If you’re proficient in PHP, you could edit your theme to allow for html in excerpts. Here’s how one person did that: http://aaronrussell.co.uk/legacy/improving-wordpress-the_excerpt/
Hopefully one of those solutions help. Good luck!
