Since the Instagram API won't send posts from a specific user with a specific hashtag, the plugin works by retrieving a user feed then removing posts that don't have the correct hashtag. So if you have a feed that has a lot of posts and not many that have the "#yum" hashtag it will need to load a lot of posts to get 12.
The API only allows 33 posts to be sent at a time and by default, the plugin is set to make a maximum of 5 requests. So if you don't have 12 posts with the "#yum" hashtag within the last 165 posts, you won't get a full 12 posts to display.
Hopefully that is understandable as to what is happening! Here is what you can do. There is a setting to increase the maximum number of api requests on the "Customize" tab. So you can set the "max concurrent api requests" setting to 10. This will load 11 posts as of right now with the correct hashtag. Since this will cause the feed to take awhile to load, you might want to set the cache to be set longer on the "Configure" tab with the "check for new posts every" setting.
The other solution that might work better is adding a unique hashtag to the photos you want to display and displaying it as a hashtag feed. For example, instead of #yum you could use #Rocketbakery_yum and use this shortcode:
[instagram-feed type=hashtag hashtag=#rocketbakery_yum]
If there happened to be a post that wasn't made by you you could remove it with our moderation tools.
Sorry that there isn't an easy fix for this It gets a bit tricky when you have a lot of posts to filter through. Let me know if you have any questions about any of this!