I’ve had similar issues with Flickrgram and in the end had to abandon it - which is a shame because it’s so nearly brilliant for my purposes.
First of all, check you have the latest version. It’s no longer called Flickrgram, it’s now called FlickrCMS1 and is a single stack. Try updating to that if you haven’t already done so.
If it still doesn’t work, check with Flickr that each and every image you want to display is available in in the lightbox size you want to use - small, medium, large and original.
It took me ages to get my head round this but - for example - if you select ‘Medium’ lightbox and there’s one photo in the album that isn’t available in medium size, then the lightbox won’t display for any of the photos.
Flickr lists the sizes of your uploaded photos - here’s the link Yuzool gave me to demonstrate this.
In the end I flipped back to my old favourite Ultimate Gallery stack and fiddled till I got it working. Still wish someone would duplicate that stack’s features. But now I just sound like a busted record.
I’m sure Michael @yuzool will be along soon to help if this doesn’t point you in the right direction.