DHTML Spreadsheet WP 3.8.1

Hello DHX
What this spreadsheet is an excellent tool for interactive use on web pages. I have been enjoying using it on my site. Unfortunately I have encountered a problem which I hope you can help me with.
One of my spreadsheets does not retain the data entered. It had worked fine up until this problem developed. Now if I paste a new spreadsheet into the page it either changes a number of cells to “null” or fails to retain any data in some of the cells.
I have tried manually entering data into a new spreadsheet and copy/pasting it into the page and opening a completely new sheet ID and starting again, but always with the same result.
A second spreadsheet operates perfectly, even when I delete all the data and copy and past from a new spreadsheet back into it.
There are no formulaes used in the sheet, it is simply dates and text with background colour added.

I hope you can give me some guidance, thank you. :question: