CSS Issue? Google grabbing original publish date instead of republish

I’m not exactly experienced in this realm, so I don’t know the proper terminology for everything (which is probably why I haven’t been able to resolve this issue myself), but I have a blog with posts that I update regularly so that their content stays fresh. Once I update said content, I republish them and change the publication date, both so they move up to the front of my blog (for regular readers), and so that Google “sees” that they have been updated.

However, recently, I’ve noticed that Google is grabbing the original publication date, even after I have changed the publish date on a post.

For example, a post originally published in 2011, but updated 12+ times since, most recently in May 2015, is showing a Google search snippet preview with that original date of 2011. My assumption is that people searching Google are biased against clicking this post due to the 2011 publication date (I know I would be), even though the content was fully updated this year.

Here’s an example of what I mean: https://www.disneytouristblog.com/mickeys-very-merry-christmas-party-review-tips/

Unfortunately, on Google, here’s what it looks like: https://www.disneytouristblog.com/wp-content/uploads/2015/10/Screen-Shot-2015-10-31-at-3.10.43-PM-copy.jpg

I *think* this change might have coincided with when I changed themes, and I’ve tried to get support from my theme creator, to no avail. Hoping someone here knows what I can do to fix this. I’ve seen a fairly precipitous drop in traffic from Google, and this is the only explanation I can think of as to why.

Thanks in advance for the help!