SoftBank Group Corp.'s Masayoshi Son became technology's biggest investor over the past year, taking major stakes in ride-hailing, e-commerce and semiconductors. But he doesn't get much credit for his investment acumen.

SoftBank trades well below the value of its assets, which include equity in public companies like Alibaba Group Holding Ltd. and Yahoo Japan Corp. The gap has actually widened in recent months to the point where SoftBank's market capitalization is less than half its holdings, worth at least $180 billion. In the past year SoftBank has seen little change, while its stake in Alibaba alone increased by about $60 billion.

The discount frustrates executives at SoftBank. They track the changes on a weekly basis and are now planning to step up efforts to close the value gap, according to a person familiar with the matter. They plan to make more transparent how the planned $100 billion Vision Fund will benefit SoftBank and how its startup investments are valued, said the person, asking not to be identified because the matter is private. Investors are desperate for more clarity.