I have had some similar situations this year.
The Canada birds we hit up in Sask in mid October were finishing early & often...though as the hours went on the birds were less & less prone to finish, multiple circles (2 mojos, 75 to 100 snows & a dozen mallard shells). 345 to 530pm heavy action, the later it got the slower it got.
I had a hunt 2 weeks ago, stubble field, nice south wind, saw a few birds (maybe 50 working this field where i got permission & a couple hundred working the field I didn't get permission further north)..went with 2 Mojos, 2 dozen standups mallards in 2 pockets out front, space to land in between & goose shells helping conceal our blinds...these birds finished like nothing I've ever seen....barely circling once & coming on a bee-line from miles away. Morning hours were the best.
Had a hunt a couple days ago where we spotted THOUSANDS of ducks in a corn field with snow the night prior....good NW wind, light snow, 4 blinds, 3 dozen standups & 2 robo's, some geese mixed in...these birds would NOT finish hard...had a couple that would but out of 28 ducks shot we had only 3 doubles (& we weren't missing - average shot was 30+ yards). It was super strange...middle of the day too. didnt leave a pocket in middle for them to land in.
I agree that mornings are best...I firmly believe in using the mojos...I think that Northerners are more likely to finish, just as hungry ducks are...I believe that the smaller groups you can get out of big flocks are the way to go (maybe you'll get lucky & pull in a 50+ group), dont pass up on finishing birds just cause a big group is behind them, wind is key, geese help I think to conceal your blinds...but mostly I dont have an answer because I've had a surplus of scenarios where certain tactics have worked in one place & others in another...friggin mystery boys, just my two cents