older topiary buxus will have bare stems near ground level. regular untrained buxus will also tend to have bare lower branches. seems to happen when upper branches shade lower portion of shrub causing leaves to slowly yellow and drop off. may be hard to induce new growth on old stems. when shrubs are actively growing and you feed regularly with fertilizer, try to prune branches to grow in direction of holes. cut just above leaves that face in the right direction and new shoots should grow in to fill holes. will take time and patience since buxus are slow growers. good luck.
big john