header image size correction

Hello everyone, I am using a header image with the recommended size that was given for this theme, however when applying image and publishing it get cut down in width. recommended sizing is 1680x720 for this theme. Not sure why this is happening, and would like someone else's input about this matter, nothing I have tried yet seems to be working. I've tried larger & smaller sizing with the same result. Thank you in advance for your input. Tom