This behavior is related to the restrictions on the number of loaded pictures. We have a default restriction of 100 pictures, for better plugin performance. So the plugin will pull in 100 latest posts by your source username or hashtag.
So the maximum number of images is not going to be more than 100, and in some cases you might get even less. Using Exclude by Username filter will further restrict the selection of images in your widget. If you set your source, and the filter, the plugin will pull in 100 latest images from the source, and apply the filter to these 100 images.
Important!!! The filter is not applied to all the images in your Instagram source, it’s only applied to the latest 100 images.
Thus, chances are that you might not get the 100 images if you use a filter.
Unfortunately, the limit of images option cannot be changed at the moment.