Answer: Since they were planted at the same time, I would expect there to be some environmental differences causing one end to grow faster than the other. Growth rate can be influenced by many different factors. Junipers do best in full sun meaning at least six hours or more of direct sun, so shaded plants would grow more slowly. On a slope, the soil tends to be very well drained, but the top of the slope will be drier than near the base. The amount of water the plants receive can affect growth, as can the amount of wind hitting them. Any time the growing conditions are not equal along a uniform planting, you will see a difference from one end to the other.
Based on your description, I would not recommend trimming. I would recommend you keep mulching and weeding and be patient -- assuming the mature size of the specific cultivar you have will reach that seven foot width. Adding more plants would end up ultimately with a crowding problem, so that would not be a good long term solution.
I know it's hard to wait. You might want to double check with your landscaper to make sure there is nothing amiss that could be corrected, but it sounds like the lower light level is probably the cause.
Q&A Library Searching Tips