Matthersby:
Resistance training 6 days a week is a large load on the CNS, especially if you are training higher volume with failure sets.
It's likely that you are very close to the state of over training (termed over reaching).
I would suggest taking a week (or two) to unload. 3 days of training, 2-3 sets per exercise, 2 exercises for major muscle groups, 1 for minor groups, 10 reps. Use a relative intensity of about 60% (ie: if you can bench press 100 lbs for a 10 rep max, then use 60 lbs).
This will allow you to recover while still maintaining movement patterns (ie: muscle memory).
When you get back, I highly suggest cutting down on the volume. Lifting weights is the stimulus, but recovery is when your muscles actually hypertrophy.
Now, in order to stimulate hypertrophy, the stimulus (training sessions) must over load the muscles. Too much overload and you will not gain. Too little, and there will be no stimulus present. This is why progressive overload is so important in resistance training. Over the course of a period of time (i suggest 6-10 week blocks between unloading weeks) the resistance, sets, or intensity must slowly increase. The rest, however, is outside of the gym.
I suggest an upper/lower body split, training each twice a week.
If you have a lagging body part on the upper body...say deltoids for example train them on your lower body day.
Keep in mind, focus on compound movements. Your biceps are going to get a substantial amount of work with big pulling movements (rows, chins, etc.) that you may not need more than a few sets to overload them. Any more is just wasting energy.
Br