notice on the daily missions theres 3 options.
i had the same prob as you. solved it by doing a mission in order.
1st day do first mission.
2 day second mission.
3rd and so on.
4th first mission.
soon in less than a month i had achieved the completion of dog tags twice in a month. now it doesn't work for me but try it!
stupid idea, but thats what worked for me