Upper body boredom and plateau

Hi guys, anyone else like me having a hard time progressing in upper body strength? I usually follow along FB burn or Fb 30-type upper body vids (around 30 mins), and sometimes make up my own program for the day (around 30-40 min). Either I choose pure strength training, or a combination of strength and cardio (like kickboxing), and I focus on this muscle group twice a week. I am a 33 year old female and have never been strong in the upper body, but I've been doing FB workours for almost 4 years now, and although I've seen tremendous improvements in all other areas, my upper body doesn't seem to progress past the first "plateau" I've reached (like.. 3 years back!). Also, and maybe because of that, I find it hard to motivate myself to doing upper body workouts. I know I enjoy most the alternating and descending rep ones, but there aren't that many. Any ideas as to what I should be doing and how much of it to really increase my strength in this muscle group?