Comma button intermittently not working

When texting, frequently I can't get the Z10 to recognise the comma button. Sometimes the devide just won't let me insert a comma. It sometimes happens after the word 'and', for instance, but also other random ones. Instead the keyboard will flash back and forth between a regular and the numeric keyboard.

Anyone else getting this - I appreciate there may not be many as nobody seems to have posted the issue.

Thanks