If i may sensei,
How does one manifest the greater problem in solo training?
By paying attention.
The body is made of many parts, so ordinary movement is really a randori. If we pay close attention when we are doing mundane things in our lives, or when we are doing our taiso undo, we can learn how things work well when they are aligned and less so when they are working at odds. Sometimes this can be very subtle, so this is why we pay close attention.
The Greater Problem is in the non-doing. Within your own body-mind, this has to do with not interfering with its normal function. If you pay attention, you may notice that you are sitting in such a way that breathing is more difficult than necessary. We may practice habits that are not good for circulation of the blood and lymph. We may carry tension that interferes with fluid movement.
I know this, because I do all of these things. The discipline and the joy is in the willingness to notice, and to let go of patterns which do not serve. The Greater Problem is in the continuing discovery of the right application of non-interference.