How many files are you trying to export at a time? The frequency of the problem seems to increase with the file count.
1,2 or 20 seems to make no difference
from my phone! 0432 970278
That is a new one. Usually the error happens when you are trying closer to 100 files.
What OS are you running?
How much RAM do you have?
See this topic in the official Adobe feedback forum: Lightroom 6 does not export all selected images. That thread contains workarounds that have worked for others. Please add your vote and details of your problem to make it more likely Adobe will prioritize a fix. And please continue discussion there so that everyone benefits.
Yup not having bridge running seems to help. Cant say for sure yet if it "fixes" it. Problem is I usually start working on images as they export. This means that I have to wait for an export to finish before I can start work (as I assume that the problem will be there no matter what previewer I'm using). Bridge is ready for an update I think.
Please add your experience to the thread I linked to above.