Hi LeeAnn and welcome to the forum.
It sounds like these things work for you sometimes and not others accross all of your projects. Is that correct or are some things working all of the time and others none of the time?
Are you working in a network environment. Are any of the files that fail on some part of the network? (And if some fail and others don't are any of the successful files on the same parts of the network?)
In V9 there was an odd situation that produced what sounds like similar symptoms and it related to the settings for the default printer on the PC/Workstation. See the thread in this link.
I am not aware of such a problem being reported with V10 but it's an easy enough thing to check out just in case.
Has anything changed on your PC/Workstation or the (assumed) network at around the same time as the problems started? (Obviously you may not know about changes on the network ...)
Could the problems have anything to do with file sizes or output record sizes? (Speculative question only since I have no idea how large the data set are. From your description I would think this is not the central issue but you never know ...)
Finally, have you tried changing the options for the Excel version to export to to see if that makes a difference? (This may not be an option for you if you are using some of the latest formatting features.)