Mine were terribly sluggish to post, prompting me in the past to write more reviews than necessary, to jog them into action.. but I think if you write more reviews than the 6 permitted for 1,000 miles, the extra reviews do not count towards the next month's miles.
The December miles turned up on the last day of December itself! Yesterday, 500 miles for January posted. So they are coming in, but intermittently. Better to write those reviews in the early part of the month if the offer continues.