There can be so many reasons for long bouts of highs..
1) illness or infection
2) as said... Stress
3) not making changes to basal or bolus if winter affects your levels
4) in case of being a pump user, faulty equipment, sets blocked by muscle or blood
5) using bad injection sites and not trying other places
6) insulin being stored at wrong temperature
7) guestimating carbs
8) changing diet
9) not having confidence to adjust doses
10) newly diagnosed and treatment not assessed fully to lifestyle
11) rebellion against diabetes
12) slow colonic transit
13) not making sure meter is calibrated and working properly
14) eating foods that have high spike levels
15) faulty insulin pens (should be checked regularly)
These are just a few