Missions not spawning

I have my mission speed set to Realistic, to get one every 3 minutes, And missions are not spawning correctly. Sometimes they will spawn every 3 minutes like normal, then other times I'll get one mission, then for 15 minutes nothing will happen. Then another one will spawn, then nothing for the next half hour. Then I'll get an hour of normal every 3 minutes, then it'll go back to not spawning again.