I was working muscle groups twice a week for quite a while. Then, I thought I might be overtraining, and cut it back to once a week. I had the same problem. I started plateauing on almost all my lifts, but I didn't have that problem when I was hitting them twice a week.
I just started hitting my arms and upper body twice a week again. (Once hard and light the next.) Haven't been long enough to know if it's gonna solve my problem, but it seems promising so far.
The bottom line is, everyone is different. Give both a try and see what works for you.
.02,
Joker