export issues - files show exported ok but I can't upload them

I have a gallery i have exported approx 1200 images.  I have another 300 I am tyring to export.  I have not changed any settings. They show exported ok.  But when I try to attach them to an email or upload them to Pixieset they show error.  In an email it shows attachement failed.  Help! I have tried to export again - to a different location. Nothings working.