Problem in foreign currency valuation

Dear all

I'm encountring a problem during the foreign currency valuation fagl_fc_val.

we've already valuated the past 11 period with no problems at all, but when tring to run the valuation for period 12 in 31/12/2015 it creates no postings at all, althought it creates postings in test run if the program run with date 30/12 instead of 31/12. please note the following:

- exchange rates are maintained daily and checked ob08 and found no problem at all

- account determination for the Gl accounts are maintained correctly and ran with no problem for the past 11 periods

Could you help me where else could the problem result from?

Thanks a lot