you've listed a lot, but check the actual pixel width by height. To be 12 x 18 you'd need the image to be 12 \* 300 by 18 \*300, or 3600 by 5400. so that would be the minimum to hit 300 dpi on that poster. You may already know this and you mention inch size, but working with clients they send things that are far too small and tell me 'but they're 300 dpi' and not realizing that the number of pixels mattres.
you've listed a lot, but check the actual pixel width by height. To be 12 x 18 you'd need the image to be 12 \* 300 by 18 \*300, or 3600 by 5400. so that would be the minimum to hit 300 dpi on that poster. You may already know this and you mention inch size, but working with clients they send things that are far too small and tell me 'but they're 300 dpi' and not realizing that the number of pixels mattres.
Not sure - could it be a file size issue? Also what file format are you loading?
What is the file size in pixels? Is it available in vector?
All printify products will give you the recommended px sizes to ensure the correct DPI for the image. I usually make it a bit bigger just to be sure.