If I create an impress show with LibreOffice on ubuntu, and then save it as a PPSX so that I can give it to a Windows user to run in office 2010, office tells me that it can’t open the file. If I use the old pps format, all is well, but it would be nice to use the new format as who knows when the old format will be no longer supported.
This occurred in LO 5 and still occurs in LO 6
Can you assist with what I may be doing wrong?
You are not doing anything wrong. The matter is that the support of formats for M$O 2007-onwards is still problematic, especially for PowerPoint files. So far, export your files to the old formats. If you can extract a problematic slide, try submitting it via Bugzilla, this may help the developers enhance the support.
Even a better solution is to persuade your peer to use LibreOffice 
My experience is the opposite: support for OOXML is better than old binary formats (but for PP formats, it might be different). Anyway, everyone would feel that “this format support sucks” when data used by that person is treated wrong way.
But what is important is to file bug reports. This is the action number one, after which other advises are good.