Hi Mark,
Check weather your system controlling period is open for posting date period in your posting.
Tcode: OKP1
Select the fiscal year
Select the version most likely '1'
You will see the period's listed up side down, with all types Transaction that take place from Fi to Co.
For testing, just unlock period for all types of transactions.
Hope it helps.
Thanks in Advance.
Pankaj Bansal