OMD EM5 Night Portrait Help!

Initially the night portrait mode worked beautifully.  So when the background is dark, the iso will be automatically bumped to a higher level so that the shutter speed is reasonable (e.g. iso 1600, spped 1/20).  After a short while, however, the iso seems to be stuck at the lowest value (200, 250, and no higher), and the exposure time is increased to, say, 1 second or so.  I send the camera back for a replacement.

However, the replacement camera exhibits exactly the same problem!  Judging from the serial number and the condition, this replacement camera is pre-owned and manufactured more or less at the same time as the original one I sent back.  For the test, I did not change any settings.  I only insert the battery, the memory card, attach the flash, attach a lens, turn it on, select the Night Portait mode, and shoot.  The iso is also stuck at 200 or 250, with speed at 1/4 seconds or more.

Did I do something wrong?  Is there a limitation on how the Night Portrait mode will kick in?  Any suggestions on testing it properly?  Any other suggestions?

Thanks!