Need Lightroom help please.

I'm new to using Lightroom. I have Lightroom 2.7. I have a folder of images that I imported into Lightroom and made adjustments as needed. I'm now trying to export them into another folder so that I can send those adjusted files to a lab for prints. When I export them...some of the files do not get completely exported and the end result is images that are half gray. I have tried to re-export the same folder several times but it always does the same thing...but not always to the same images. There are about 350 images in this folder...I have even tried to do half of a folder at a time but it does the same thing.....why????? Is it something I am doing wrong?

Ok...I guess I need to update this...I think the whole conversion is there but it just does not all show up in the file folder. When I view the suspect images in different programs the images show up ok. Strange!

Well, it's hard to comment -- can you show us anything to show us the problem?

Please clarify: Do you mean that you have ticked the checkbox on the Export page so that the exported jpgs are imported into LR and then they appear corrupted in Library? But in another application they are ok? If viewed in Develop, how are they?
I have never used that reimport function because any jpgs that are to be sent off I delete right after sending them.