Hi Glen,
Most mall bonsai are seriously underpotted to begin with, but if you consider that conditions in a typical heated & air conditioned office or home environment are as dry as a desert and darker than Pharoah's tomb, that amount of soil can't meet the tree's requirements on an ongoing basis.
Check out the article over on
TreeBay Bonsai Tools & Supplies on mallsai and consider moving that tree outside. It's fine for display for a few days indoors each month, but sunshine, rainfall and ventilation are the factors that are going to keep it healthy in the long run.
Based on the information you've provided, if this tree were any species other than juniper, it would be dead already. Juniper are uniquely adapted to dry conditions, but they aren't cactus, so they can't take drying out for extended perriods. Beyond this, they do need sunshine.
Best regards and feel free to ask any follow up questions!
Matt