If the meter says flash is only about 60% of the exposure, ambient light is still a big part of the reading, which can make troubleshooting confusing. The simplest check is to remove ambient from the equation as much as possible and test where flash is nearly 100% of the light.

Try this:

• Use a darker environment or settings that minimize ambient.
• Keep distance and all camera settings fixed.
• Take a meter reading, then change flash power by several stops and meter again.
• Also take actual test photos at the same aperture.

If flash power is really changing, the images should clearly overexpose or underexpose as you raise or lower power, and the meter should reflect that.

A useful cross-check is guide number math: at a known distance, set an expected aperture from the flash guide number and compare your test shot and meter reading.

If neither the photos nor the meter reading change, the most likely issue is that the flash is not actually changing output even though its controls suggest it is. In short: isolate the flash first, verify output with test shots, then compare that result to the meter.