Exporting from Lightroom

Although only using lightroom for 12 months,(on a Mac) I have become confident in using for my purpose.   Saving images, cropping and changing light etc.  I have exported them successfully, with the changes,  and gone onto use in prints, photobooks etc.      In the last few weeks I have been unable to export an image after making changes.  The image exports as the original, without changes.    I have read through this help community and watch tutorials and following instructions.   Despite doing everything as I should the image is not exporting with changes made.    I have read some of the help topics and must confess some are way over my head.     Is there a fix to this problem?

Many thanks