[Plugin: Custom Field Matrix] Uploading images doesn't create different sizes

Normally, when I upload images to the media library, or even with other similar plugins such as ‘Custom Field Template’, wordpress automatically creates a thumb, medium and full size in the upload directory. When I upload images using this plugin it uploads the original image size but does not create these other sizes.

How can this be done?

Thanks

https://www.ads-software.com/extend/plugins/custom-field-matrix/